I had the exact problem on my 190E, but the sound was there only on idle speed and it came and went, but only when cold, once it warmed up, you never heard it again. It was the tensioner shock for the serpentine belt, I ordered a new one and replaced it as well as corrected the way it was installed since it was not done correctly. Never heard the noise since that time.
If anyone is interested, what I noticed is that the tensioner would vibrate very aggressively when on idle speed, but it smoothed out on higher rpms, that vibration was what caused the noise combined with the worn tensioner shock.

xp

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1985 190E 2.3L - a constant project.