2000 C280 with 58,000 miles on it has this loud THUMPING sound from the center and passanger side vents. The ac and heat blow fine but this thumping sound is driving me crazy. also when I turn it on and off there is a boom from the passanger side and the other sound is like"spring"sound that goes off. Of all my years of owning Mercedes I have never came accross anything as weird as this. Is this normal? PLEASE HELP!

 

One of the linkages on the stepper motor (behind dash) has broken.
You should replace the stepper motor and the link rod. Depending on which one it is, could require dash removal.