When I replaced the evaporator and 6 vacuum servos on my 1993 400E, I found the thin foam padding on vent door atop the heater box badly deteriorated. I removed the rest of the foam that had not already fallen off. After reassembly, I noticed that the noise from the vent door closing was much more noticeable. The failsafe mode for the system is with the defroster on so when the engine is turned off, the vent door opens to defroster mode. When you restart, vacuum pulls that door closed with the resulting clunk. I suppose the vent door could be replaced or new foam installed but it took me about 6 hours just to get face to face with that door.