I would suspect the belt is getting worn, becoming hard and starting to slip going around the pulleys. Especially if it has some age on it. Also look for cracks in the ribs on the belt.This happens every few years in my 95 S-10. If it is a new belt possibly a wrong size not fitting well on the pulleys?
If you're certain that the sound is from the serpentine belt you should check the belt tensioner. A weak tensioner spring will result in slipping and a noisy belt.
the belt is ether too old, or a faulty component that is being driven by the belt.
Check the allignment of all the pully's.
You don't. The belt tension is automatically adjusted via a spring loaded tensioner. If the serpentine belt is noisy or worn out prematurely. replace the tensioner. The belt should be inspected for cracks and other signs of wear. Replace worn out belt(s) before they break leaving you stranded.
it could be a serpentine belt or accessory belt.
No, an accessory belt is not considered a serpentine belt. However, a serpentine belt is considered a specific type of accessory belt.
If it's a "chirping" noise, then you don't have the belt tight enough.
The serpentine belt would be the same as the accessory drive belt.
Serpentine Belt Diagram ML320
Serpentine belt, no. Timing belt, good chance yes.
If the alternator belt is the same belt as the other accessories, yes.
Almost every car ever will not start without a serpentine belt. The serpentine bell acts similarly to a pulley. It's movement causes everything else to rotate and move. If you look at an engine, there's a number of circular shapes where the serpentine belt goes. It uses traction to rotate them.