This might be stupid... but my uneducated guess would be that the isx only has two bells (like every other engine) one for the fan and the other one to turn a gazillion pulleys... personally i would loosen the bells and start shaking them for play.
You are correct. A fan belt and a sepentine belt. Idler and tensioner are both on the serpentine. They're the most common suspects because Cummins thought it was a good idea to make a part that needs to be durable out of both metal and plastic with no way to grease the bearings.
Glad you found the problem. Hope it's not too much of a pain to fix.It's not the belts or pulleys had mechanic check all and listen to the noise it's unfortunate but it's my air compressor :-( it's stuck on open I guess and it's making that Insane noise. We took off the air intake for the compressor poured little transmission fluid and it stopped making noise for about 5 min, also noise is gone when air compressor is building air. For example if I dump all of the air noise is gone while it's building, but once the air reaches cut off point noise kicks in and it doesn't go away!
And btw we took the belt off, noise was still there