I have noise, which sounds like a fan blowing air coming from the passenger side when the car hits 40MPH and above. I first thought it was the air cabin filter, so I installed a new filter. Alas, same noise. Is there something that kicks on at 40MPH or just coincidence the noise starts at that speed? This happens whether climate control is on or off.

Appreciate anyone's insight on what could be causing this noise. and more importantly, how I can stop it.

Thanks in advance,

 

You shouldn't hear the fans in a C280, Most likely it is a loose piece of trim on the windshield or the rear outside view mirror. Get some masking tape and mask the trim on the windshield and around where the rear view mirrow attaches to the body. Since you wrote the noise is there with the ACC off. I bet it is some loose piece of trim. Tape one trim piece at a time until it stops. Also, the grille could be loose and vibrating at 40mph and above. Also the air intake grilles at the bottom of the windshield check to make sure they are seated?