Even new belts give sounds and have to be re-adjusted sometimes. When checking belts, a slipping belt usually has shiny sidewalls (glazing) caused by heat from sliding in the pulleys. The glazing often causes a chattering, slapping or squealing noise.
The glazing hardens the belt, it's prone to cracking and early failure.
Belt slippage often occurs because of low tension, the wrong belt, or grease or oil.
Missing chunks, or separating layers of the belt often hint at a dousing of oil or grease. The source of the oil or grease needs to be corrected before installing a new belt.
Belt with any of these conditions should be replaced as soon as possible.
On serpentine belts, inspect the belt closely for any cracks in the ribbed area. Also check the backside of the belt for grooves, fraying, or splitting.