I have a 2001 silverado and just recently i started hearing strange noises coming from the right air vents, it sounds like something is stuck in the vents and is trying to open or close and makes a knocking sound. does anyone have any idea what it could be?

 

There are motors in the system that control the air vents, the AC/heat and the defroster. One of those motors is probably stuck and trying to move. They are mostly made of plastic and tend to break with age. I have already replaced the 3 in my truck. Check to make sure that the Defroster works and that it changes from hot air to cold air

 

the recirculation door actuators like to go out might be a place to start looking