An easy diagnosis and fix. Take the tension off the belt (via a 3/8 ratchet and the belt tensioner) and remove it. Then, reach in and spin all of your idler pullies by hand. If they're loose, or sound like they're full of sand, the bearing is worn out and needs to be replaced as it is likely the source of your noise. While the belt is off, inspect it visually. If there's any noticeable wear, rips, or cracks in the belt, replace it. If not, put on a pair of rubber gloves, get yourself an old pickle bucket and go to town on it with a scrub brush, some scalding hot water and some Ivory dish soap.
While the belt is off, this would be a good time to inspect your water pump and power steering pump to ensure they're not leaking and leaving any residue on the belt.
Assuming all is well with the other components, reapply the belt (or install the new on if necessary), start it up…bring it up to operating temperature and take if for a ride.