I am trying to track down a rhythmic grating sound from the belt system on my 1991 300CE, 137m. Using the screwdriver as a stethoscope, all is smooth at the alternator, air pump, compressor, water pump, p/s pump and either end of the fan pulley bracket. I located and eliminated a bad tensioner shock. I do feel a little roughness when the screwdriver is on the belt tensioner. There still seems be enough belt tension because it still took plenty of muscle to take the belt on and off when I replaced the shock. The history I have shows belt tensioner replaced at 87m, 9/1998.

Anywhere else to check?

glenmore
1991 300CE
2000 C280
1990 LS400