Belt rubbing tensioner

Don't recall how that one is setup, but on other trucks I've had the alternator wobble in it bracket enough to push the belt out of line.

Something like that would be a good place to start. Some pulley should look a little off from the others in the line when viewed from the side.

 

Might have a bearing going out on a pulley or idler and it is causing the belt to ride funny.

 

You did get the belt routed properly?

 

Anybody put a straight edge across the pullies and verify their alignment?

 

I’ve seen idler and tensioner pulleys wear into a cone shape after many years of use. They well walk the belt off one way or another. Worst one i’d seen pushed the belt back towards the front gear housing causing it to slowly cut through a bolt head like a band saw blade. Seems to happen more often when the belts are able to attract a lot of dirt and they polish the smooth face of the tensioner pulleys.