Ok...This is a 1990 300TE...I have been messing with this stupid problem since I got this car...about 3 months now. When you remove the glove box, you can see a vacuum diaphragm module (whatever it is called) mounted to the center vent structure. Looking in from the glove box area, this is mounted on the left. You can see that it is attached to a lever that runs inside this vent structure, and that its purpose is to open an internal vent door or something. Now, when you press a vent or ac button, you can see this module suck in, pulling this lever with it, and all is well. When you press the O button, you can see this module release out. Periodically, or intermittantly, when you press a vent or ac button, you hear this "thump, thump" sound coming from inside this center vent structure. You also see that the side module has not sucked in like it should. If I apply a little force to this module, I can get it to suck in, but the "thump" noise is still happening. Then the next time I start the car, no problems....works just fine. Now, when this happens, I tried to vary the fan speed to see if it altered the timing of the thump thump, and the speed made no difference whatsoever, so I believe we can rule out the fan. I think it must be some kind of vacuum leak, or possibly a bad vacuum module located inside this vent structure. This thump, to me, sounds like one of this modules getting stuck trying to suck in, but that does not explain why this outter one that I can see does not work when I hear this thump noise.

The only thing I have noticed, and I am not sure if this is 100% accurate...but when I start the car, and the vent does not make any thump noise, it will continue to work fine the whole time. If I start the car, and there is this thump noise, it will continue to thump until I start the car again. Now, I have not yet tried to kill and start, kill and start, to see if it will go away doing this, but I will try it the next time it happens. So far, it happens about once in 5 starts.

Any ideas, or suggestions for troubleshooting this problem. I plan on selling this car with EVERYTHING working properly in the next couple of months.
Thanks