Anything that varies the drive load on the belt will make the tensioner "jump" -- it's a rubber "spring", so any accessory (or a bad belt, for that matter) will make noise and vibration.

A bad belt can sound just like a rod knock.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!