Sounds like a pulley has a worn bearing. Are alternator, A/C, water pump all working OK?
You can try spraying WD-40 or lubricant into all pulley shafts/bearings to see it helps.
One time my engine was squeaking. I replaced the serpentine belt and it went away.
Or try to locate the source of the racket with a listening device like a stethoscope, or a rubber hose to your ear. Be careful dont let the hose get caught by the belt and choke you to death.
