1994 E-320 heat and air?
 

Hello everyone!
For the last couple of months when I turn off the heater or ac in the car I hear a loud flap or thump which I think is the door that redirects the air. I am curious if there is some type of vaccuum device that controls this and if it might have failed and a good starting point on where it might hide?
Thanks in advance for any help.
Dan

There is some foam on the rear of the door that is decaying. When this happens, it exposes the glue. Then the door jerks open and closed instead of moving in a fluid motion.