86560SEL, it may be a failing fan bearing bracket that is making the noise. I had that experience after a mechanic replaced the water pump in our 1991 300CE, due to the drive belt being over-tightened. If you are not a DIY'er replacing the fan bearing bracket requires stripping down the front of the engine ... quite expensive if you're paying a mechanic. If that's the problem buy a reconditioned fan bearing bracket from a MB dealer, rather than some cheap knock-off brand.

I would take the car back to the mechanic and get an explanation for the noise. There are many things that could be making the noise. In any case you would expect the noise existed when the mechanic finished replacing the water pump, and didn't suddenly appear right after you got your car back.

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Fred Hoelzle