In December I had a new heat pump installed, and since I've begun running the a/c, I've had a strange issue. After the a/c shuts off, it starts to emit a high pitched whining/buzzing noise of varying loudness and tone for a few minutes afterwards. It's loud enough that I can hear it in a quiet room of my house, but not very loud. It seems to be coming from the air handler in the area where the line set goes into it. Completely shutting off the electricity to the system makes the sound go away.

Some web searching seems to indicate this may be a normal sound of pressure equalization in the line set, but I never heard it in my old heat pump, and it seems strange that cutting the power makes it stop. Does anyone have any clue what this might be? Is this sort of sound normal? I don't want to go calling my HVAC company if they're gonna tell me there's nothing to be done about it.