I'm doing to replace the main drive belt on my 35D as I have a periodic squeak at startup and I'm at 100k miles. Does the tensioner or idler pulley usually fail/squeak? Or is it both? I'd rather not replace parts I don't need to on this car.

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2012 X5 35D - EGR delete and Stage 2 tune
2003 M5 - Imola/Caramel, Dinan S2 suspension, Alpha-N tune
1990 M3 - CF Airbox, Alpha-N, Schrick 284/276 cams, VAC S2 head, full GC suspension
1991 M5 - Black/Black - stock
1991 325iX - Winter beater

Retired: 1999 328is 5MT, 1994 525i 5MT, 2003 530i 5MT, 2012 X3, 2006 325i 6MT, 2001 M5