knocking sound when AC on
 

I have a 1989 260e and experience a peculiar knocking sound coming from the air vents when the AC is on. I've never had this system apart and don't have the manual that covers the climate control system. I'm thinking that it may be a flap type door in the system that is vacuum actuated and not remaining closed. The system seems to work fine; the air blows cold. Any ideas anybody. The sound is more of a nuisance that anything.