Could be a lot of things. I used a gatorback belt to replace the OEM one and had LOTS of sqealing because the drive pully was slipping with the AC on or the Power Steering turned hard one way or another. I was able to fix it by getting an OEM belt, then cleaning all the pullies with a wire brush and some brake cleaner. If that doesn't work, you might have some damaged or misaligned components.
Spray hairspray on the belt if it is making the noise or replace the belt or the pulley if its making the noise.
Water from the road has splashed up into the engine serpentine belt causing belt slippage that makes a squealing noise. The sqealing is one of your engine assessories that use the serpentine belt to drive it.
A worn belt will cause a squealing noise. The squealing noise is caused when the belt slips. Tightening the belt will temporarily fix the problem.
If that's the extent of the complaint, check the serpentine belt and tensioner.
Check the condition of the accessory drive serpentine belt, belt idler pulley and belt tensioner pulley bearings.
Serpentine belt. If its cracked, frayed, or worn replace it before it breaks or you're in for a hefty repair bill.
Change the belt
It sounds like you need a new serpentine fan belt.
Most likely a worn serpentine belt. Replacing the belt with a new one should correct the noise. If not the belt, it could be the idler pulley, which is not putting enough tension on the above belt, causing it to slip and thus squeal.
Check you serpentine drive belt and pulleys.
Nine times out of ten it is a worn serpentine belt. Tensioners are also know to make a rattling noise congruently. Changing your belt and/or tensioner should solve your problem.
You need to be a little more specific about the noise around the belt. I would suggest you check out the serpentine belt as it could be failing. If it is a grinding or squealing noise it could be a failing bearing in one of the units the belt powers. I hope this is of some help, if not ask again with more information.