How do I diagnose an accessory belt noise?

One day recently when it was a bit cold (60F or so) and wet, my belt squealed a bit.  I didn't think much of it.  It went away a few seconds after startup.  A few weeks later, it happened again, only much louder.  Then something 'clicked' and the sound changed to more of a bearing/pump squeal.  And it doesn't go away.

Standing with the hood up while its running, I can't really locate where its coming from.

I took the belt off and checked all the pulleys, they all seem to rotate freely and don't make any noise.    Then tensioner has a tiny bit of wobble if I try to 'twist' it by hand.  How can I verify it is the problem before replacing it?

This is on a FWD, so the accessories are right up against the RH frame rail.  I can't really get in there with a stethoscope on any of the pulleys.

AC clutch going bad?  

Well, I took my traditional approach:

Step 1:  Post thread

Step 2:  Immediately find the problem

I went back down to the garage to put the belt back on and try and do some more diagnostics and... the tensioner had like no tension.  It was loose enough that I could install/remove the belt by hand.  It must have been weak and just gave out when I loosened it to take off the belt or something.

I chased after a noisy belt or accessory problem on my truck. I replaced the fan clutch and the tensioner before I figured out it was the alternator bearings. 

I came here to post about pulley noise, being late at night i read pulley instead of belt and i'm just going to comment anywa.

The best way to chase a pulley noise is remove the belt and spin pulleys by hand while checking individually with a stethoscope. 

glad you found your bad tensioner. 

I wonder if one could make a belt-spinner using an old pulley and a spare belt fastened to a high speed drill.

That way you can spin the individual pulleys up to speed and see what noises you hear or don’t hear.

I know it is too late but next time spray some WD-40 on the belt, if the squeal goes away it is belt/tensioner related. WD-40 increases the traction of the belt

Well, now with the new tensioner it is nice and quiet *except* when accelerating from a stop with the A/C on.  I get a squeal for like 2 or 3 seconds.  I'm wondering if the belt is dead as well, or if the new tensioner is faulty too

ProDarwin said:

Well, I took my traditional approach:

Step 1:  Post thread

Step 2:  Immediately find the problem

Hey, if it works for you, keep doing it!

I gotta do the same thing on the Impala SS.  I think mine is bad bearings in the new A/C compressor.  Not only does it tick me off that its the new compressor, but they are a royal pain on the LT1.

If you have a steady idle and can measure the diameters of the pulleys, you can use a sirometer. That is if the pulleys are different diameters.

http://www.treysit.com/english/

I do have to admit, while I have a sirometer, I've not had the opportunity to hunt down such a problem with it.

I know you figured your issue out, but in the future, some earmuffs and a section of 3/8" hose works wonders.  Put the ear muffs on to drown out the squeal, but stuff one end of the hose under one of the ears.  Point the hose at each pulley and it will pinpoint the noise.

This works great for lifter ticks, exhaust leaks, or any other noise you can't otherwise find.

I've used WD40 with a straw to find the noisy pulleys before. spray them at the bearing one at a time to see if the noise changes/disappears. 

Curtis said:

I know you figured your issue out, but in the future, some earmuffs and a section of 3/8" hose works wonders.  Put the ear muffs on to drown out the squeal, but stuff one end of the hose under one of the ears.  Point the hose at each pulley and it will pinpoint the noise.

This works great for lifter ticks, exhaust leaks, or any other noise you can't otherwise find.

Good trick!  Thanks.

Right now the plan is to replace the belt.  It only has ~6 years, 70k miles on it, but it is a cheap Dayco so who knows.  Also it may have taken some damage due to the bad tensioner(s).  Now to find the right size belt... everywhere I look online lists 2 different sizes for my car.  778 and 780

I know the OP mentioned restricted access, but my old homemade steth approach (long piece of hose to the ear) has helped me find some noises lately including a dried up lower tensioner pulley on my Jetta 2.5 and now a small exhaust leak at the flex pipe (this one was very tricky to spot after spending some good time under and over the engine bay).   Obviously common sense with regards to any moving and/or hot parts come into play here.