# Heater/AC is hissing while fan is on



## cranberry521 (2 mo ago)

Over the summer my AC stopped cooling and while the fan is on it had a hissing sound. I let it go and now that were into heating season its doing the same thing while heater is running. When the fan is off it doesn't make any noise. Any idea what this would be?


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## snowwy66 (Nov 5, 2017)

Mine 17 has made a noise since day 1. 

5 years ago.

Barely noticeable. I'm used to it now.


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## jblackburn (Apr 14, 2012)

Certain heat settings will attempt to run the AC compressor still. Defrost/floor, floor/recirculate, and of course defrost itself.

Hissing heard from the interior s usually an expansion valve noise


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## Kalevipoeg (3 mo ago)

Cars switch conditioner on also during cold season when air is blowing to windshield, to defrost/dry it. Also cars sometimes switch system occasionally on, to ensure cooling system and conditioner pump lubrication.

Weird hissing sounds are caused by low pressure of cooling agent in system, and it is too low because something (like some connection o-ring seal) in system is leaking. Faulty seals must be detected and fixed, and then system re-filled.

Another option, unplug conditioner pump connection and disable this system forever. In this case getting it again working later is even bigger headache. But you can use this method to check whether my assumption is corrrect.


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