Intermittent clapping noise from ventilation

I have a 1991 300se (W126). Recently the ventilation system has been making an odd noise. About one to two minutes after the A/C has been turned on, a noise that sounds like a large book being closed seems to emanate from deep within the center part of the dash board. According to my wife, the heater also is not working properly (now that she is back in South Carolina I have not worked up the motivation to verify this in 95 degree heat!).

Any idea might cause the noise? Would it be related to a poorly functioning heater?

Sincerely, Eric Powers

[dpetryk]

There is a large movable panel that is moved by a vacuum actuator that selects the air source. Its movement selects either outside air or inside air (recirculate). The panel is covered by some type of foam insulation that degrades over time and becomes sticky. It is really bad if the driver is a smoker. This panel sticks in the closed position until there is enough force from the actuator to move it and then it makes the noise you hear. It is located behind the glove box and can be easily accessed by removing the glove box. A little cleaning in the right places will solve your problem.