When you have the car stopped and in drive, the position of the engine will be ever so slightly different than it is in neutral due to the torque loading on it. This can account for the difference in noise for the serpentine belt tensioner.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record