I would guess the aircon system (having thought about it) as it changes low pressure gas into high pressure gas, so at any one time, whilst operating there are 2 diferent 'pressures' in the system maintained by a pump of sorts.
On shutdown, as the pump is no longer turning, this pressure (i guess) equalises throughout the system, and that may be the noise that's heard?
Only a guess mind, I could be completelly wrong.