this will sound weird, but spray water on one pully at a time while it is running, if the sound disapears when spraying a particular pully, thats the bad one

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1996 E320
2000 C230 Kompressor
1988 190e 2.3 - 225K miles, owned for 7 years. I cannot say enough good things about this car. Very well built, even at 225 it ran like new.