VERY LOUD slapping air vent upon cold start.
 

Something has changed with the air-handling system in my 1995 E320. When the car is cold, the air vent closes with a VERY LOUD slapping sound. It seems like the vacuum is too strong or too weak to manage the internal vent that controls air flow in the heating system. This started when my transmission work was done. I wondered if there was a connection with the vacuum for the kickdown actuator and the air-handling system’s vacuum. When the car is warm, the slapping is not nearly as loud. It always closed during cold start-ups, but it was never so loud. I think there must be something wrong with the vacuum controls for the internal vents that control airflow. I am hoping it has something to do with the transmission work, because then the dealer may fix it as part of a warranty service for their work. I am hoping it is not a problem with the “pods”.

Please help me understand the problem so I can help the dealer identify the problem.

usually there is a foam padding on the fresh/recirc door. the foam maybe missing or flat and this is allowing the door to audibly slam closed

Thank yo for the tip.
 

I did know about the foam and I guess it is possible that the sound is only louder because the foam is messed up, although it really seems like there is an issue with the vacuum.

This is no help, but
 

I have exactly the same situation. When I click my key on to glow the car in the morning after it's parked outside ('95 E300D) I get a big slam as an air vent closes (or opens?). Basically I just decided not to worry about it, as otherwise my system seems fine.

Rgds,
Chris W.

I was thinking about that, but....
 

I am just afraid it might damage something that would be much more expensive to fix.

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.

Too much suction
 

I wasn’t sure if I had a leak and less vacuum or too much vacuum. Based on your description, if vacuum pulls it shut upon start-up, then it seems that I have more vacuum after the transmission repair. I really think there is some adjustment with the vacuum that has my car shifting a bit oddly when cold and that also affects the vent closing. I have just a very slight flair with shifts between 3 and 4th gear. The dealer adjusted something and it helped the flair a bit, but it still does it a little too. The slapping vent started with the throttle actuator repair, so I am convinced that they are related. It is possible that it is just a coincidence, but I think there is a connection.

My 1993 300E 3.2 does exactly the same thing. There is an loud annoying flap from the dash when the car is started. Leaving the control in normal mode with the fan on low when shutting down the car alleviates the problem when you later start the car. My transmission also shifts somewhat abruptly between 3-4 when cold. Is this an indication of a potential problem or something that is a simple adjustment? Just purchased the car 1 month ago. 92,000 miles.