I mention this because my belt made a slapping sound whenever it traveled across the air pump pulley. Sounded like the bearings were going out on the pump.

Upon inspection, it was found that a piece of the toothed side of the belt had chipped off. The smooth side looked brand new. Replaced the belt and the sound went away!

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car