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Date registered: Nov 2002
Vehicle: 2008 ML550, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
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The air pump has a pulley bearing that spins any time the engine is running and a set of internal bearings that spin only when the electromagnetic clutch is engaged which is only for the first few minutes after start up (yes, you are carrying around a lot of weight that is used only about 0.01% of the time). If the whining you hear continues after the start up period (5 minutes or so), it is most likely the pulley bearing. In general, this is the most common failure on the air pump.
Bearing replacement is a fairly easy DIY. I bought a replacement bearing on Amazon Prime, had it the next day, and two hours later, all was well. I know the part number for my E320 (M104), but I don't know if it is the same as the pump used on your car. You will need to research that.
Hope this helps,
Floobydust: a contemporary term derived from the archaic Latin miscellaneous, whose disputed history probably springs from Greek origins (influenced, of course by Egyptian linguists) - meaning here "a mixed bag." - National Semiconductor Audio Handbook