This is going to come as a shock to most of you, but there is something ELSE wrong with my '05 H2 with 34k miles.

The only vents that work are the defroster vents (ones that blow onto the bottom of the windshield).

None of the other vents work. Not sure if its related, but whenever I turn the car off, a groan comes from the dash like someone is punching a senior citizen...

Any suggestions as to what it could be?

 

An air vent control valve that is binding? Many years ago I guy I knew had a Chevy wagon that had some kind of automatic vent control. It was deformed and it bound. Instead of groaning it let out a loud bang when it would finally close.

 

check the vacuum system. Sounds like a hose has come loose. Just fix where it came loose and she should be fixed.

 

I doubt they are vacuum operated any more. Most use small electric motors to control the air flow and the temp blend door. If you can loacte the sound I bet you will find a small black box with stripped grears in it. They are not very expensive and not too hard to change if you can reach it. Most mount with two screws and one electrical plug.