That's exactly what it sounds like. Been there since day 1 and I just assumed it was a vacuum release or something.

 

Both my 2008 HHR SS and our 2010 Camaro RS do this when you shut them off. The Camaro is not Turbocharged so I doubt it is the wastegate.

Mike

 

It's actually the throttle actuator cycling.

 

Mine doesn't plunk, but one time it hissed real loud then I realized I had parked on the cat's tail.

 

Mine plunks even if you never start the car, just turn the ignition key from ON to OFF.

 

I had the engine running with the hood up. Went over turned the engine off, and heard the "Plunk" right behind the air box. Took a quarter out, dropped it over the back of the engine, and "Poof"!, damn thing propositioned me. "Moon"

 

its the throttle body plate closing

edit: my old cobalt did the same thing...and it was the 2.2...that i turbocharged....then we also have a saab 9-3T and it also makes this sound...its the electronic throttle plate de energizing

 

exactly not vacuum controlled as stated a few pages back..its nothing related to the turbo or wastegate...its the electronic throttle body!!!

 

yeah perhaps.