I went through a semi-enclosed drive-thru the other day (Yes, I know! Bad!) and was astonished at how noisy the serpentine belt had become on my 15 year-old ride. Come to think of it, pretty girls had been looking at me strangely when I drove by. Hmph. Thought it was me. Time to take action.
Got the stethoscope out (a piece of fuel line) and with the engine idling and while being hyper-aware of the rotating parts, I isolated the alternator as the culprit.
Took the belt off and gave the alternator a whirl by hand. Hmmm. Sounded OK.
Well I decided it was so easy for me to remove it and have it bench tested that that is what I'd do. It tested fine.
Many years ago this rig squealed too. That time it was dry idler and tensioner pulley bearings. Stupid vendor was chintzy with the grease! A normal person would have replaced them right? Well I found a small syringe, loaded it with grease, and was able to work the tip under the seal lips and load 'em up good. They still sound like new.
Back to today: All the rotating bits were good and the everything passed visual inspection too. So what th'? I thought, maybe it's dusty/dirty/greasy enough to squeak? Made a nice strong batch of hot soapy Dawn water and scrubbed the belt and pulleys down. Rinse, dry, install... no change.
I gots online and found tons and tons of bad advice- throw new parts at it, fool around with shims, put belt dressing on it, put GREASE on it?!?
But then I found a page that made sense: after eliminating all other causes, the pulleys and belt are probably GLAZED.
Glazed? But I already inspected it all! Can't be! With nothing to lose I got out the wire brush and brushed the belt down (heh heh, you didn't think I'd listen to the page and go get a new one?!?) and all the pulleys too (except portions of the main drive pulley on the crank that were too hard to reach). Since there was a good chance my brush had oily residue on it I cleaned everything up with brake cleaner.
Voila! Success! This old dog learned a new trick. Glazing on V-belts is easy to see but I simply couldn't tell by visual means on the serpentine belt and pulleys. If you're having a tough time silencing a noisy belt system this might be the ticket.