I was getting out of my patrol car (07 Impala) for lunch today and heard a very odd noise about one second after I turned it off. It sounded like something was thumping behind the air bag and neither me nor my sgt could figure out what it was. The car ran fine, all the gauges were in the norm, didn't even make the noise when turned on or driving, though every time I turned the car off for the rest of the afternoon, it did it. After trying out a few things, I figured out it would only happen when the ac was turned off, either manually or when turning off the car. I played around with it some more and found what was making the noise. When I would turn the ac on, there is a thing I can only describe as a plunger that moves upward and out of sight (I am only able to see through the half inch gap between the back of the glove box and bottom of the dash). When the ac goes off or is turned to defrost mode, the plunger moves back down and appears to get caught on something and not seal/lower fully. It will then still try to move down for about 45 more seconds, making the thumping nose, before finally stopping. I know it has something to do with the ac and am assuming there is something that is blocking it from returning to it's usual spot. Does anyone have a better guess as to what's causing it and how to fix it without ripping out the glove box?