Fan belt noise from the engine compartment of a car can have many causes. In order to diagnose fan belt noise, you need to isolate where the noise is coming from to know what you may need to have fixed. The fan and other accessories in an engine are typically operated by the main serpentine belt. You can diagnose fan belt nose by visually inspecting the belt or checking for a bad bearing on a pulley.
Check whether the belt appears frayed. A worn or frayed fan belt can have some of the outer fabric come off, which can come into contact with parts of the engine and make a noise.
Look to see whether there are cracks that have developed. Cracks in the belt can cause small pieces to break off that can make noises in the engine.
Examine the belt to see whether there is a small stone or other small object in a groove of the belt. Any object stuck in a belt groove will cause a noise when it runs though the pulleys.
Check whether the belt appears wet. A wet belt will not make proper contact with the pulleys and can cause a squealing noise.
Take the vehicle to a mechanic to see if the belt tensioner is going bad. A bad belt tensioner can lead to a loose belt, which can cause a noise.
Place chalk around the edge of one of the pulleys used for the serpentine belt. The chalk will appear to brighten when the pulley turns.
Start the vehicle and then watch the pulley as it rotates.
Listen for a sound and see whether it occurs at the same time the chalk brightens on the pulley. If the sound is heard every time the chalk brightens, the pulley probably has a bad bearing.
Replace the serpentine belt if you see cracks or any other indication that the belt may be bad.
Failure to replace the fan or serpentine belt can prevent the alternator from charging the battery.