Heat Pump Noise

What would cause a Heat Pump to make a brief whistling noise when it is not running…refrigerant pressure equalizing?

That is what I always thought. It should only do that shortly after turning off though.

Kind of weird Bruce, it is one of mine but it doesn’t necessarily make the noise when it just shuts off, it might be 10 minutes latter, and not all the time, just once in a while.

It works fine, but I noticed it making the noise again tonight and thought someone might have a guess.

Equalizing is what my guess is…but I could be wrong.