If the noise is from the expansion valve opening, that is completely normal and sometime occurs on cars while the AC is in operation. Do a web search for how an AC or refrigeration system works.

My 97 SL320 and 97 C 280 make expansion valve noises while the AC is in operation. Both valves are new, the SL was changed due to it sticking shut, the C because I had the under hood duct work out to change the windshield and the valve was easily accessible. ( I don't wait for parts to fail, I change them when they are at their expected end of life. )

If the noise if from the vacuum control system as others suggested, I don't have specifics on your car. I have seen various cars have a failed vacuum check valve that allows vacuum to bleed off once the engine stops. When the system defaults to "Hot" and "Defrost" , sometimes it makes noises.

Another possibility is the brake booster developing a vacuum leak. A test would be to run the engine, shut it off then wait 60 sec or so, press the brake and see if you still have power assist for 2 or 3 presses. ( Pedal pressure will get progressively higher after each press. )