RE: Buzzing, whirring sound with headlights: Here's what it is.
I think I know for sure where the noise is coming from. I have the same problem on my 1990 300E. Whirring sound when headlights turned on, whether engine running or not.
I have isolated the problem to being the headlight wiper motor. Neither one of my headlight wipers works. The driver's side is frozen (can't even move it by hand.) The right side appears to have a broken shaft between the motor and the wiper arm. When I turn my lights on, the wiper motor runs constantly, at high speed. That makes the buzzing sound.
What baffles me is that the wiper motor is only supposed to come on with both the headlight switch and the wiper switch on. I have not taken the components apart yet; my plan is to just disconnect the headlight wiper motors and keep the wipers for decoration.
However, if anyone knows why this occurs, (i.e. could my headlight switch be bad,) then I would like to know. Please note also that on the same side as the bad wiper motor, the fog light comes on without me having pulled the headlight knob out to put it on.
1990 300E 125k Miles
1994 S420 100k Miles