I am sure someone out there has had this problem before. I have a '99 ML430. When it is cold outside (below 45*F or so) the HVAC works as it should. Temp controler dial turned to hot blows hot air, temp dial turned cold blows cold and anywhere in between.
However, when the outside ambient temperature is warmer there is a clicking sound from inside the dash when the temperature dial is turned to cold. As the temperature outside goes up the less I can turn the temp dial into the cold side before it starts making this steady anoying click. It makes the sound regardless of recirculated air or if the AC is on or not. When it is really hot outside 100*+ the dial does not go into the start of the cool side without the click. If I run the AC with the dial as far into cool as I can get it before the click starts after a few minutes I can turn it a little further before the click starts.... wait a while.... turn it down some more... wait a while... turn it down a little more...and so on. So by the time I reach my destination the dial is almost to the coldest setting and the air is really feeling good. As the car sits in wait for my return everything heats up again and the clicking returns.
It has acted this way for the past couple years and I have been searching for answers to this phenomena off and on for a while. Now it is to the point where I understand what it is doing and just deal with it because it is so predictable but I dread the day I have to explain this quirk to someone else. Oh, no it's charecter!...Really! Summer is comming up and it would sure be nice to enjoy the cool air in quiet.
Thanks for any suggestions as I do not know the first place to start looking anymore. I have replaced the cabin air filter with the HVAC on and it sounds like it is comming from that housing (I can feel it). I am sure it is the sound that would come from a flap constantly occilating. I have not been frustrated to the point of tearing the dash off in search of the problem or spraying adhesive into the system to stick whatever is flapping shut or open (J/K) but I am nearing that stage (doh... then there will probably be a humm or buzzz to go with the click and then a cha ching $$$ from the dealer) Glad you guys and gals are here to help me diagnose these things or at least rationalize my frustration.

Thanks,
Loren

I'm having the same issue with my '00 c280. I can hear the oscillating click from behind the dash. I *think* it may be a leaking vacuum flap actuator that is slowly opening the flap, then it gets to a point where it can no longer hold a vacuum, slams the flap shut and repeats.

I'm sure we're not the only two on this forum who is experiencing this anomaly, so hopefully someone who's found a solution will chime in soon..

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2000 c280 Sport (looks like an upgrade, feels like a downgrade)
1991 190e 2.3 (sold)