Belt squeal

Most likely the tensioner. Some manufacturers sell a small plastic gauge to check the grooves on the belt. The belt can wear but not have cracks. Best to replace belt and tensioner and the squealing should go away.

 

The tensioner is not only for tension but also has a damper in it. If it gets really bad it can ruin the bearings of everything the belt runs such as alternator, ac compressor, water pump. Not a very complicated job. There are videos on you tube. If you decide to do it yourself make sure to check and clean out the pulley grooves.

 

Coolant can soak into the rubber if there are leaks or has been leaks or accidental spillage on them. Do not use over the counter belt dressings.

You can clean all the pulleys with soap and water and thoroughly dry them and just replace the belts with new ones.

If they are still in good shape and you don’t want to replace them. Try applying a small amount of silicone based grease. Brake caliper grease works good. A little goes a long way. I just smear a glob on an inch or 2 of each belt and then run the engine.

Best method of cure for belt noise I have found. Course this is assuming belts and tensioners are in good operating condition.

Do not use petroleum based grease.

 

The pulleys wear over time and no longer fit a new belt. The alternator pulley
wears the worst because its the smallest.

 

from the Gates website (one manufacturer of belts and tensioners). I basically stated this in post 2 and 4.

Our research has shown two main causes of belt noise:
1. Loss of tension in the drive: This is typically indicated by a squealing belt at engine startup. The belt squeals because there is improper tension in the belt drive system. This is usually caused by a worn automatic tensioner, but other causes can include a contaminated belt (oil, belt dressing, etc.) or a belt that is too long or worn out.
2. Misalignment: A chirping belt typically indicates an alignment problem

The two primary roles of the automatic belt tensioner are to: 1) apply the correct amount of tension to the serpentine belt as it transfers torque from the crankshaft to the system accessories and 2) smooth out crankshaft vibration associated with the transfer of power.

A tensioner is a relatively inexpensive part to replace and can help protect other components, such as the water pump, alternator, and A/C compressor, from undue stress and premature failure.

While the engine is running with the air conditioning on, check the tensioner arm for excessive vibration. If the tensioner arm vibration can be seen with the naked eye, then the vehicle needs a new tensioner.

 

Based on all the excellent answers, I would buy a new tensioner. As usual, I would keep the old working one for a spare. Good for an emergency if bearing fails on new one, or diagnostics if bearing starts making noise. Till then, you can get a paper bag, cut 2 holes in it, and wear it over your head till the squeaking stops. Lol