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Could be the belt, idler pulley, tensioner, or any one of the other pullies.
Best course is the start it and go and look/listen. I used an extension bar to push on the tensioner to see what that did to the sound. Found bad tensioner, idler pulley, and a balancer that had walked off.
On a high mileage vehicle I'd shoot the parts cannon and replace the idler, tensioner, a/c tensioner and both belts. I like the green HD Gates Fleetrunner belts.
You could also eliminate one tensioner by swapping to a stretch belt for the A/C like the GMT900's.
Part # shown is what fit on my 2006 6.0; you can skip that installation tool. Didn't work in my application. I just used zip-ties through the crank pulley and pulled the belt on turning the engine over with a breaker bar, no big deal.