My approach ...
Inside or out side the engine? Remove fan belt while recording its path, leave access panel off. Run engine for only 20 seconds. If same noise, inside engine. If not then next step.
With belt still off, hand spin all idler pullys and all auxiliaries (water pump, alternator, a/c, etc). See if any of them wobble or sound funny when spun. When spinning them by hand, spin them only in the direction they would spin if connected to the belt.
Reconnect belt using the diagram you made before taking it off.
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