Your best bet is to get a mechanic's stethoscope like this and carefully (as in avoiding belts fans and other rotating items) listen to each idler pully while the car is running.http://www.harborfreight.com/c...41966

You could also remove each pully and listen for roughness and grinding when you turn them by hand - but my way avoids disassembling and reassembling the car with a bad pully while you wait for the part to arrive if it's not immediately available.
Heath
1990 Infiniti Q45 **** AKA "Q2"
2005 Infiniti G35 **** Sedan A/T
2004 Nissan Frontier.. King Cab M/T