fast clicking noise under dash 91 300E W124???
 

It's intermitant and sounds too fast (like ratcheting) and too loud to be a relay switch. I haven't noticed it while the car is idle. I checked under the foot rest panel but don't think it's coming from there. I suspect it's related to the ventilation but can't tell unless I disassemble the dash or lower panels...

Is the noise coming from the far right side dash vent? Do you have automatic climate control? If yes to both, I suspect the air sampling motor is dying.
regards,
Mark

how do you access and check this motor?

thanks

It's the vacuum manifold that directs vacuum to the different pods on the heater box. You can access it thru the glove box.

part number?
 

Is this the "A/C E.T.R. Switch" p/n R1067-74361? I think I need one on my 91 300e.

It's this part:

I'm having the same clicking noise, but its coming from the driver's side.

i had a similiar noise it was the automatic climate control pump,its located behind the far right vent,its a small black pump with a rubber hose comng from it,i removed mine dissasembled it oiled it then reinstalled it t.he noise went away

Aspirator Blower (It Sucks)Motor for HVAC,Removal
 

http://www.ps2cho.net/downloads/MB%20CD/W124/w124CD1/Program/Climate/83-637.pdf

AND the Sensor itself(little square grille inside the left hand side of the Sunroof/
Cabin forward light Switch array).

http://www.ps2cho.net/downloads/MB%20CD/W124/w124CD1/Program/Climate/83-640.pdf

I hear the same clicking noise, but its coming from the driver's side, and it only happens when I accelerate.

Colebob, the aspirator motor and the vacuum swithcing valve are both good suspects, as detailed by the guys above. But yours "might" be the speedometer, if it just happens during acceleration. Mine ticks just like the old cable units, even though it has no cable going to the tranny (magnetic pickup). I had it replaced while still under warranty; now the 2nd one is doing it after 15 years (acceptable I guess).

DG