124 noise at Idle
 

Between 600 and 650 rpm, there is a grating sound at what appears to be around the water pump. No leakage is evident around the pump. 700 and up, noise goes away. Coincidentally, I had the serpentine belt replaced a few months ago and the noise became evident after the replacement.
Is this the beginning of a water pump failure?

could also be (the common) fan clutch bearing bracket

with engine off - try to rock the pully for the fan back and forth - or with engine on try to (carefully) get a sthescope down nearby bases to hear better

sometimes it's better just to wait for it to fail and be ready

;)

When the noise appears, stick a screwdriver blade into the top bushing mount of this damper and twist to tighten the bushing.. if noise goes away, top bushing is bad/worn......very common..

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1M30LAGSC1VF188OT0&year=1990&make=MB&model=300-E-002&category=G&part=Acc.+Belt+Tension+Damper

I bet Arthur is right. Happened on 2 M103's I have owned. Sounds like marbles when the hood is opened.

No harm, just annoying.

Steve

Dam... now I've got it...