The Serpentine Fan Belt - Belt Tensioner & Idler Pulley.
The Serpentine Belt or S-Belt is typically a longer, wider fan belt that the traditional V-Belt. The S-Belt gets its name from the way it weaves around the different pulleys on the engine like a snake or serpent. The belt wears out and makes a chirping, squealing or knocking noise.
The Tensioner is a spring loaded arm that automatically keeps the S-Belt tight at all times. The tensioner wears out with age and will no longer keep the belt tight. it can also cause the S-Belt to prematurely fail. The tensioner bearing and spring can make many different noises or no noise at all.
The Idler Pulley and bearing also wears out with age. If either part breaks, the belt can jump off the pulleys causing the vehicle to over heat, lose power steering and discharge the battery.