This is an Idler Pulley. Each vehicle has a few different pulleys that help the engine perform functions and run accessories. This type of pulley has little ball bearings that help it roll to move the belt that is ran around it and other pulleys. When a pulley like this starts to squeak it is time to take it to a mechanic. Once it starts to squeal then it is getting really hot and in this case essentially melted the metal and welded the little bearings to it. When this happened the pulley could not turn freely and it put tension on the bolt that was holding it in. The bolt started to loosen and come out, and the belt shredded. This customer was lucky that the pulley did not come off because it would have stopped the engine, and potentially caused an accident. Please, if your vehicle is making noises that it does not normally make, bring it in for an inspection. Our Multipoint Inspections are only $45, and if you schedule repairs with us that $45 goes towards your repair.