# High pitched humming after engine shutdown



## jblackburn (Apr 14, 2012)

Many theories, many have heard the noise, but no one seems to know for sure!

http://www.cruzetalk.com/forum/10-powertrain/3384-buzzing-sound-after-turning-off-car.html


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## OldWhiteChevy (Mar 5, 2013)

jblackburn said:


> Many theories, many have heard the noise, but no one seems to know for sure!
> 
> http://www.cruzetalk.com/forum/10-powertrain/3384-buzzing-sound-after-turning-off-car.html


Wow...you're not kidding: 208 posts on this subject! I watched a YouTube video that someone had posted and it's the exact same noise I'm hearing. I had never thought of checking YT. Anyway, it's sort of reassuring to know that I'm not alone. Thanks.


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## iTz SADISTIK (Apr 6, 2014)

OldWhiteChevy said:


> Wow...you're not kidding: 208 posts on this subject! I watched a YouTube video that someone had posted and it's the exact same noise I'm hearing. I had never thought of checking YT. Anyway, it's sort of reassuring to know that I'm not alone. Thanks.


No worries, it's just the countdown of the device, but last time I heard Jack Bauer was stuck in traffic! May the odds be ever in your favor


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## earnhardtnorth (Apr 21, 2014)

OldWhiteChevy said:


> FWIW, the noise sounds kind of like what you hear from a fluorescent or mercury vapor light whose ballast is starting to go bad.


You a sparky too?!


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## Action Hank (Jan 9, 2013)

Yeah I experience it often on hot days, lots of posts on the subject but no real conclusions that I know of.
Mostly everyone just has to accept it as normal for this car.


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## Gonzo74 (Mar 1, 2014)

I was told at the dealership that it was something to do with cooling itself down. I was also told that the 1.4 engine was a toyota design that GM paid a lot of money to keep quiet, so bs was everywhere. My two cents there.


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## Psychomidgit (Jun 9, 2013)

Right after I bought this car I noticed it. I brought it to the dealer and the writer said it was and auxiliary fan. However, the writer couldn't tell me where it was, so I figured he just came up with something thinking that it would make me happy. I'm sure I'll find whatever it is one of these days.

Gonzo: I've also heard that and that it was a German design.


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## CruzeTech (Mar 23, 2012)

Everyone has their own take on what it is. I subscribe to the theory that the turbo cools itself after shutdown. It's been described previously but people seem to come up with other theories. 


Sent from AutoGuide.com App


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## ls1vazquez (Jan 25, 2013)

I have a 2012 Cruze Eco. I've heard this noise also on occasion. I assumed it was the servo for the air dam getting caught mid-cycle. I suppose not.


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## obermd (Mar 3, 2012)

It's not our air shutters.


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## consv (Dec 14, 2015)

2014 Cruze with 1.4 turbo. Random buzzing from front of engine.
I called 3 different dealerships and they all said I would have to take the car in for them to tell what the buzzing is. Wouldn't even take a guess. It used to just happen right after turning off the engine but now I am noticing it come on even when the car has been sitting for a while. If I open the front driver's door it goes off. Usually worse in the winter.


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## RIXSTER (Apr 13, 2015)

I have this noise also, I just figured it was from me being in race mode. Could be muffler bearing. jk


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## ChevyGuy (Dec 13, 2014)

consv said:


> If I open the front driver's door it goes off.


Hmmm. I think someone mentioned that the car's fuel pump pressurizes the system when you open the door. Perhaps that buzzing sound is related to the fuel system slowly losing pressure.


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## Yates (Aug 24, 2012)

I just call it Casper. It seems to be friendly.


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## 2015cruzer (Dec 13, 2015)

hey guys.. I am a tech at a GM dealership and I found your concern. It has to to with a software calibration for the HVAC control module, if your car is still under warranty you should go get it updated.

This is what I found on my super awesome GM mechanic database.. Id say its the noise your talking about.


"Some customers may comment on a buzz type noise after the vehicle has been shut off. The noise may be noticed with the A/C on and may last up to 10 minutes after the vehicle is shut off and most often is intermittent. The noise is similar to the sound of a reed valve emitting a fairly constant tone.

The air conditioning refrigerant solenoid valve is not consistently powered down at key off due to variation in drive cycle and emissions monitoring. When the solenoid is operating at a 50 to 60 PWM range and certain conditions are met at key off, it will continue to emit this buzz type noise until the system powers it down."


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## NickD (Dec 10, 2011)

Have a 2012 2LT with MT, no firmware updates are available for this car. Also never heard this type of noise in our Cruze.

OP also stated, hears this noise whether the AC is on or not. Not sure what solenoid valve you are talking about, my Cruze uses the V-4 variable displacement compressor with a fixed orifice, only solenoid anywhere in the system is the solenoid that is used for the compressor clutch electromagnet, and that is off when the engine is turn off. 

Sure had this kind of noise in my daughter's 98 Ford ZX2, but was quite a bit louder. In these new vehicles, drawing a deep vacuum in the PCV system, for the Cruze at 1,200 altitude, this vacuum measured by the oil dip stick with a special adapter is 19"/Hg, and it leaks down. In her Ford was a loose connection in the PCV hose that also had that reed like effect, but really loud.

I would suspect a slow leak in your PCV system, granted, never easy to find the source of a noise, when sound travels seven times faster through metal than air, but can be done with experience and a stethoscope.

If you hear this noise, trying pulling the dip stick out quickly and learn if it stops that quick.


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## 2015cruzer (Dec 13, 2015)

Im not talking about any solenoid, you would have to contact who ever wrote the document on Service Information to find that out. Also, if you read the whole paragraph it states, "customers MAY hear the noise while ac is on". 

edit: also just noticed this software update was avalible on july 8th, 2014 so if you have updated anything since then obviously its been updated.


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## jblackburn (Apr 14, 2012)

What he said. Definitely the compressor, definitely makes the noise only if Ac has been running.


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## ChevyGuy (Dec 13, 2014)

NickD said:


> Not sure what solenoid valve you are talking about, my Cruze uses the V-4 variable displacement compressor with a fixed orifice, only solenoid anywhere in the system is the solenoid that is used for the compressor clutch electromagnet, and that is off when the engine is turn off.


I thought the variable displacement was controlled by computer. I think that control (done via PWM) is what's being heard.


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## 2015cruzer (Dec 13, 2015)

this is shown for the ac compressor under the description and operation of a 2012 cruze with automatic HVAC.

A/C Compressor 

This A/C system does not use a conventional clutch to engage the compressor. It uses only a variable displacement solenoid valve to alter the amount of displacement created by the turning of the compressor. The HVAC control module provides both battery voltage and a pulse width modulated ground to the variable displacement solenoid valve. When the A/C switch is pressed, the HVAC control module grounds the variable displacement solenoid using a (PWM) signal in order to determine the amount of compressor displacement. The performance of the A/C compressor is based on adjusted interior temperature and engine load.


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## Merc6 (Jun 8, 2013)

#PI1267: High Pitched Whine or Buzz Noise from Engine Compartment After Engine is Turned Off - (Jul 8, 2014) 

Had it done and never came back.

This is what we were talking about 2015cruzer?


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## 2015cruzer (Dec 13, 2015)

I believe that hvac software update will fix your issue, original poster.


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## NickD (Dec 10, 2011)

2015cruzer said:


> this is shown for the ac compressor under the description and operation of a 2012 cruze with automatic HVAC.
> 
> A/C Compressor
> 
> This A/C system does not use a conventional clutch to engage the compressor. It uses only a variable displacement solenoid valve to alter the amount of displacement created by the turning of the compressor. The HVAC control module provides both battery voltage and a pulse width modulated ground to the variable displacement solenoid valve. When the A/C switch is pressed, the HVAC control module grounds the variable displacement solenoid using a (PWM) signal in order to determine the amount of compressor displacement. The performance of the A/C compressor is based on adjusted interior temperature and engine load.


Possible in Cruze models after 2012, but sure isn't in the 2012, still using a conventional electromagnet engaged clutch like the very old POA systems used in the early 60's. When the AC is switched on, runs continuously until switched off. After this, GM went to the CCOT systems that constantly cycled the clutch on and off to very quickly wear out the compressor.


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## NickD (Dec 10, 2011)

Even have more questions on this new compressor buzzing when I assume the ignition is off, older V-4 fluid rate was controlled mechanically by pressure. In the 2012, the entire MVAC system is cut off by the ignition relay, no wonder I had problems with this, have a bunch of circuits on those itty bitty contacts.

So if this newer system does buzz after the key is off, must always be hot, meaning yet another means to wake up to a dead battery. If the darn firmware doesn't switch it off.


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## 2015cruzer (Dec 13, 2015)

That exact description is for the compressor on a 2012 Cruze with automatic hvac. It is correct.. If I was working on this specific Cruze at the shop and I needed information that's the info I'd get, That's the info any GM technician in all north america would get.
domyou have manual hvac? Or automatic?

EDIT: actually both compressors are the same for automatic and manual HVAC, the description and operation of the compressor for both manual and auto HVAC are the same. I even checked the wiring schematics and they both are the same as well.


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## NickD (Dec 10, 2011)

I opened the hood on my 2012 2LT with both a manual transmission and climate control system. Only two wired going to the AC compressor clutch electromagnet.

For my over 60 year history of reading shop manuals, the 2012 Cruze shop manual sets new world records for being the absolute worse most incomprehensive, stupid, disorganized shop manual ever written. 

I do have one small advantage over my Chevy dealer service manager, least I can read. 


I do not doubt that some Cruze vehicles have an electronically controlled variable displacement compressor, just thanking whoever that I don't have one.


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## jblackburn (Apr 14, 2012)

NickD said:


> I opened the hood on my 2012 2LT with both a manual transmission and climate control system. Only two wired going to the AC compressor clutch electromagnet.
> 
> For my over 60 year history of reading shop manuals, the 2012 Cruze shop manual sets new world records for being the absolute worse most incomprehensive, stupid, disorganized shop manual ever written.
> 
> ...


You do. It's controlled by fan speed. Notice that the drag on the engine is immense at higher fan speeds and very little at 1.

Seems to also be a clutch in the front of it that disengages the compressor shaft from the accessory belt when turned off.


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## NickD (Dec 10, 2011)

Will have to eat my own words, wouldn't be much of a problem if Chevy could draw a complete circuit diagram. Yes there are two pins on the compressor connector. After some search found them.










A third wire does go to the compressor coil to enable the compressor. And yes this is an electrically controlled variable displacement compressor. GM for years used two wires for the clutch coil, now just using on, the other end of the coil is grounded. 

What controls the amount of refrigerant flow in the compressor is a thermistor mounted by the evaporator, the warmer the temperature, the greater the duty cycle of the compressor mounted solenoid valve, the more the cooling capacity. Increasing the speed of the blower motor does direct more warm air to this thermistor increasing the effective output of the compressor. Have to say in theory at least, this is a far better system than the older mechanical variable displacement system that worked on pressures.

Monitoring the evaporator temperature is the most common sense method for best control, but the Japanese were already doing this back in the early 80's. Ha, I date way back to the York compressors that had a sump for the refrigerant oil, practically all good air compressors work on this very intelligent method. But someone got the brilliant idea of making these things like a two cycle engine where you have to mix the oil with the gas, but with refrigerant instead. York's had a sight glass, one glance and you know its okay. Not so with these newer system, no dip stick, if you lose oil, only way is to completely flush it out and pour the correct amount back in. Not even oil, your compressor would be toast, too much, lose cooling, a bit too much, will oil slug the compressor and wreck it.

They also used a sight glass on the high side, one glance at 1,500 RPM, ambient above 80*F, should be bubble free, this was the second glance. No guess work. 

Did look at the difference between the Cruze manual and ACC system, (automatic climate control), in the manual system, the ignition relay completely switches off the MVAC system, with the ACC, it is always hot.

Ha, kind of had my feel with ACC for like 40 years, would either be baked alive or frozen to death. Wasn't too bad in the 60's, could pull the entire unit out and set it on my work bench. Just a one transistor circuit, using a vacuum pump for power could see everything work. This became history with the micocontroller, but its much cheaper, but also good drive a guy nuts trying to find the problem. Okay, so I am nuts, LOL, not my fault. 

So never want an ACC system again, well my 88 Supra has one, but does have a manual override, so can live with this. Besides, I am tied up like a criminal anyway behind the wheel, and not really a great effort to make minor settings to the manual control, and a heck of a lot simpler.


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