idler pulley...

 

That's an expensive lesson but at least you know you have a new alternator! Keep the other on the shelf as a spare.

A mechanic's stethoscope is a great tool to have - Harbor Freight sells them dirt cheap. Or, just grab a long screw driver or metal rod - put one end on the mounting bolt for each pulley with the engine running. Careful to not get into the belts or fan. Go back and forth between each pulley and you'll hear it when you hit a bad one. I still suspect its your idler pulley on the driver side, people usually chase this problem for ages until they give in and replace that one, which very often fixes it. They don't always make noise when you spin them. It wouldn't be a waste of money to simply replace them all.

The part that would worry me is that it's making your truck "run rough". How so? Are you sure it's the whine that's causing it to run rough?

 

It was my alt. Changed it and sound went away