I have a 190e 2.6 with a bird-like squeak coming from the belt drive system - I believe. Here's the history:
Original water pump failed at 86k. Replaced with a Graf unit. Also replaced the thermostat and rib belt (Conti).
Filled with some genuine MB coolant. Followed the factory service manual steps.
At 90k, it developed a random 'squeak' coming from the water pump area. Got my stethoscope out and searched for the noise. It was not clear. Suspected the tensioner pulley. Sprayed a little lube on it and it seemed to quiet down for a short period. Heard some noise around the bracket, so I pulled everything off to replace the pulley. It was worn, so I replaced it and the idler as well. After pulling it off, I turned the water pump a couple of times and heard a slight noise, like a squeaky rubber bushing - so, I replace the Graf with a genuine MB pump, and put it all back together.
After a short trip (1000 miles) the squeak is now back in full force. Stopped into a Benz dealer in KY and had a mechanic look at it with his scope. Did not hear anything in the water pump - heard some noise in the tensioner area. After explaining my replacement story, thought that it could be the tensioner failing. Belt tension was okay.
He suggested replacing the shock and the tensioner - both could fail and cause this type of noise.
Car runs excellent with perfect temp in hot weather.
Any experience with this type of failure? I am trying to nail this before a replacing another part.
'93 190E 2.6 92k
'81 240D 209k
'89 Range Rover 140k