My M104 idles quietly, but one time I did notice a rattling sound and the tech pinpointed the belt tensioner as the source of the noise.

Replaced it under warranty at 85K.

It's been quiet since then.

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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