When my A/C turns off after a few minutes it makes a screeching sound that lasts about 20 seconds. You can hear it about two houses down it is so loud. My unit has a heat pump, and is about 5 years old.

I have had four HVAC people look at it and no one can figure it out. Two of them said they had never heard anything like it. I had the reversing valve changed about two weeks ago and the noise stopped for two weeks and now it is back.

Other than the noise, the A/C works great. What could be causing the noise?

  • You say it can be heard 2 doors down, I assume the noise is coming from the outdoor condenser unit? If the noise only occurs when it is spinning down then the first thing I would inspect would be the condenser fan / fan motor. Bad bearings or debris perhaps?
    – CactusCake
    Commented Jun 8, 2017 at 13:27
  • Is the noise coming from the motor as it is spinning down, or from elsewhere after the motor stopped?
    – ajeh
    Commented Aug 18, 2017 at 13:48

2 Answers 2


It could be a leaking discharge valve or reed on the compressor. When the compressor shuts down the head pressure pushes past the valve. If the unit has service valves, shut off the discharge valve out of the unit, run the compressor until the head pressure increases to it's max, shut off the compressor and listen. Any good service tech can do this. If there are no service valves, then I would run the unit, shut off the compressor and use a stethoscope to listen to the compressor. If it is not the compressor then use the stethoscope on the reversing valve. Check the fan bearings to make sure it is not the culprit. And while you are at it go find a good service tech and never call the other 4 imposters.

  • I made a typo; I said shut off the suction valve when I should have said discharge valve.
    – d.george
    Commented Jun 12, 2017 at 9:30
  • Good answer a little chunk of crud in the valve could make this noise. Similar to using a blade of grass between fingers to male a whistle. The system should be checked for acid in the oil and of that is ok a new filter/dryer installed , if acid is indicated the compressor may have problems but this is doubtful because it stopped for a while.+
    – Ed Beal
    Commented Oct 20, 2017 at 14:49
  • Is it a dangerous condition if the Compressor makes this squealing noise? I left my home for two weeks and when I came back my condenser fan motor wasn't working so my compressor was running none stop (I had thermostat low). Afterwards, I noticed the squeal. Could it be caused by being run so hard?
    – LeanMan
    Commented Nov 20, 2021 at 4:39

It's a compressor reed valve leaking refrigerant backflowing and equalizing pressure during the off-cycle. It seems to be an unrecognized problem among a/c people! There doesn't seem to be a fix except to replace the compressor. I've had two units do it and it can drive you crazy! I hope someone comes up with a reasonable solution!!!

  • This doesn't provide any additional information that isn't already included in other answer.
    – JACK
    Commented Jul 7, 2023 at 20:51

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