The fan clutch engages using a bimetallic strip. When the engine gets hot, the strip flexes and locks the fan to the shaft. Otherwise it will spin freely and save some power. An elegant solution to a difficult problem IMHO.

A fan clutch going bad should not cause the sound you describe, nor should it cause the belt to slip. Mine went bad and can caused the clutch to be engaged all the time. The only symptoms were that the fan was really LOUD and the engine ran really cool.

I would look at things like alternator bearings or the water pump.

__________________
"It's not about how fast you can go, but how well you can go fast."

Bob in Richmond

'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles