I have noticed a chirping noise coming from the serpantine belt somewhere on the rotating assembly of my 1990 190E 2.6 200,000 miles. I don't hear it all the time, it is more noticeable when its damp outside. I'm thinking it is either the tensioner pulley or the idler pulley. I've tried to listen to where the noise comes from, but can't seem to pinpoint it. It sound as if a bearing on one of the pulleys is making the noise. Any suggestions on helping locate the fault or typical failures would be appreciated.

this will sound weird, but spray water on one pully at a time while it is running, if the sound disapears when spraying a particular pully, thats the bad one

Spraying water on a serp belt might not be effective because the single belt will quickly transport the water to all of the pulleys.

I found a mechanic's stethoscope for $13 at my local parts store is incredibly sensitive and can pinpoint noises very well. You also don't have as much danger to self as you do with the old "long screwdriver or combination wrench to the ear" trick.

After you've found your problem, you can use the stethoscope to listen to your whole engine and see what's sounding right and what isn't. Injectors, valves, air pumps, water pumps, anything that moves in the engine has a "normal" sound signature that you can monitor and recognize when it starts to go bad.

JDUB:
 

I have a similar noise aswell on my 2.6
My car has its original belt having travelled 126 000 km.
I was told at the dealership just to change out the belt and the tensioner. I was quoted $200.00 for the job without the tensioner so I've waited on getting the job done. How many miles (or kms) did your original serpentine belt last?

A chirping noise points to the alternator or the water pump if it is a screetchy type of chirping. Good luck with everything on your 190E!!!

Squeeky belt
 

I recently had a similar problem on my 190E 2.3. I solved it by replacing the belt. After the fact I also replaced the tensioner assembly, as a separate issue. I've had the car for only 3,000 of its 140,000 miles, and didn't realize it wasn't supposed to sound "like a diesel" at startup. In my sttemp to locate the source of my "squeek", I also found my "rattle".

A short length of ordinary garden hose works well in a pinch as a substitute for a stethescope.

I replaced the belt last year at about 170K. Fun job! The belt seperated and I had to extract strands of the belt from the fan clutch. I had a mechanics stethascope,but you know, (kids). Thanks for the input.