Serpentine belts are a great invention but unfortunately they drive everything on a modern engine which places a lot of load on them. When your belt displays any of the following:
-squeals when accelerating.
-squeals when wet or moist outside
-squeals when cold
-or any combination of the above.
The belt must be replaced, but wait… If you replace the belt it may stop for a while but then the noise comes back. This is because the old belt was slipping and has polished the pulleys or "glazed" them. So before you replace that noisy belt scrub all of the pulleys well with a scotch brite pad, or a good wire brush to remove the glaze and rough up the surface. Clean them off with brake cleaner afterward. Belts work by friction so if the surface is polished too much the belt will slip. If you have already put on a new belt and it is making noise, -sorry you must buy a new one again. Slipping on the pulleys will glaze a new belt fast which ruins it. Some people say you can apply brake cleaner etc to a new belt to fix it but that may not work well. If it slips it will just mess up your newly cleaned pulleys and you will have to scrub them up all over again. So do the job right and rough up the pulleys AND replace the belt at the same time.
There are a few other things that can cause serpentine belt noise tho so before you replace the belt check the following:
-The bearings in the tensioner and idler pulleys can go bad. To test this put some chalk or whiteout on the edge of those pulleys so it makes a bright "flash" you can see when it rotates. If the chirp is in time with the flash on one of the pulleys most likely you have a bad bearing in that pulley. Sometimes if you have the right tools you can press in a new bearing. Other times you need to get a new assembly. Other accessories can have bad bearings also so you may need to mark and watch them also. WARNING: Never stand in line with a moving belt or fan. If it comes apart etc it could cause severe injury.
-The tensioner can loose it’s tension which will cause the belts to slip. If it is easy to move the tensioner then it needs to be replaced and the pulleys cleaned and belt replaced.
-The belt can become delaminated and part of the outer fabric cover can come loose which will "flap" when it hits parts of the engine making a noise. If you hear this tick tick tick noise and it seems to be in time with that printing on the belt then this may be the problem. Always install a serpentine belt so that the cover splice is pushed down by the motion of the belt rather than being peeled upward.
-The belt can crack or delaminate and sections of the inner VVVVV surface may break off and cause mysterious noises. If you see cracks in the inner surface replace the belt.
-Hope this helps!
REMEMBER: NEVER PUT ANY LUBRICANT ON A BELT!!! A lubricant may stop the belt from making noise temporarily but it defeats the purpose of having a belt. You need FRICTION between the belt and pulleys to transfer power. If you lube a belt it changes the frequency of the sound the belt makes to one you can’t hear but will not drive the pulleys fast enough and you may end up with a dead battery, dim lights, poor AC, or high engine temps, etc. The slippage and lubricants work together to produce heat in the belt which will destroy the belt rapidly.