How do you isolate which pully is making noise?

[RobPatt 93 525ia]

I've a rattle/squeak that definitely sounds like it's from one of the pulleys in front of the engine.... it's not the compressor as I took that belt off. Remaining pulleys are all ran by one belt. It's most pronounced with warm engine, in gear, w/brake on.... Belts 2 years old. Help - it's driving me nuts!
Thanks. Rob. M50TU.

[Tiger]

Stethoscope to each device to hear the strongest rattle. It could be the tensioner itself. Otherwise, I'd take the belt off and wiggle and spin each pulley until one is loose and rattles slightly.

[Derek A.]

Might just be the belt. I was convinced I had a bad water pump, turned out to be warn belts. spray some silione on the belt, if the noise goes away than you know its time for a new one.