Here is a good way to locate the offending pulley. ironically I'm heading to get mine done at the garage this time but I thought this might help.
Have had strange sound come and go and couldn't find it then just recently the sound happen while stationary.
Unable to tell but was convinced it was brakes or the cv axle.
Probably the auto idle pulley now I've had gotten to hear from the outside.
Edit :- Was the water pump in my vehicle.
Also the method in comments below would be a great idea if you have a them laser gun. To check for temperature differences between pulleys.
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