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!!!

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1989 300CE - 269,000 km
1994 SL600 - 59,000 km