Is it a squealing noise, or does it sound like something got into the fan blades?

When I was working on my 300SD, I found insulation had gotten ingested from somewhere and was stuck in the squirrel cage fan, as well as having dry bearings.

If it's just the squealing noise, try dribbling some ATF into them (down the blade of a screw driver works fairly well, once the motor is on the work bench...)

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...