Vent direction problem
I have a 96 C280 that did the same thing. I did a few things and finally located the problem. (Assuming you have the LCD temp control unit above the radio)
It turned out to be the control unit connector. Get two slim peices of metal to slide into the from of the unit, remove your glove box so you can use your hand to push the control unit out of the dash from the rear of the unit. Use the metal "keys" to release the clips that keep your control unit in place.
Slide the unit out, there are two connectors I believe. There are a little tricky, there is a release arm that when pulled away from the unit, pushes the connector out of the socket. On my unit the pins on both the socket and connector where corroded. I used a small metal brush and some contact cleaner to remove the corroson.
Push the connector in and out of the socket several times, then give the system a try!
This fixed my unit. It would act the same as you discribe and make a strange deep wind instrument sound too... foggy windows and all.
Before I figured this all out I also disconected all the Vaccum lines from the solenoid control module located behind the glove box. It's easy to find because all the vaccum bladders that change the position of the routing fins have a thin hose that goes to this unit.
So if you have worked with a volt meter you can pop the lid on the soleniod control and measure the coil load on the 8 solenoids, or disconnect the unit and trigger each solenoid with 12v. (Don't worry about damaging it, no IC circuits just a plain old Solenoids.)
I hope this helps! My motivation was 97 degree weather in the summer with all the AC blowing on the windshield and heat routed into the car!!! So on top 97 outside I had the heater stuck on!