Easiest thing to do is obviously to replace the belt. I had the same problem where the belt was getting noisy but was not really visibly worn. A new belt solved the problem. It is also realtively easy to change on n M104 engine. Probably take about 30 mins and $30 or so for the belt.

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Ali Al-Chalabi

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