If I had to venture a guess, I'd start looking at the starter bendix not fully disengaging or something in the tranny. It's definetly rpm related, and follows the engine rpm. If you don't figure it out straight away, I'm sure it will rear its ugly head before long.

 

Im a tech at a chrysler dealer. This sound is usually a bad idler pulley or tensioner pulley bearing. Take the belt off and spin the pulleys. They should be quiet and spin smoothly, but they should stop spinning pretty quickly and not go on for days.. Also try to move the pulleys side to side to check for play. If you hear any roughness in the pulleys AT ALL they are junk.

 

I agree with a few others, one of the pulleys (alt, ps). Not buying the belt unless it is "ribbed" for the trucks pleasure.

 

My truck made the same noise it was a bad alternator bearing replaced alternator noise was gone