Try removing the serpentine belt, start the engine, and see if the noise goes away. If it does, most likely a pulley problem or the alternator pulley is going bad.
Inspect the alternator pulley by removing the plastic cap on the end of the pulley and look for red rust dust.
While you have the serpentine belt removed from the engine, hand spin and feel for any excess play or roughness in all of the pulleys bearings including the idler pulleys, viscous heater pulley, AC pulley, mechanical fan pulley (spin fan by hand), etc...
If the noise continues with the serpentine belt removed, most likely the harmonic balancer has gone bad and rattling.
Only option if bad is to find a good used one as new ones are no longer available anymore or send your balancer to Damper Doctor and let them rebuild it.
DamperDoctorThe #1 source for new and rebuilt harmonic balancers, Dampers for internal combustion engines. vibration dampers rebuilt to OEM specs.
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