Is it happening at idle or under load

Perhaps if possible can you post up some footage so we can have a listen

 

Pull the belly pan and use a long screwdriver or wooden dowel to 'listen' to the sound by placing it on the alternator. I'd also look at the other belt idlers. Shouldn't be too hard to isolate with the hood open.

 

Ranchero is right, that is a sure fire way to determine the source of the sound. However, unless you have experience in doing this, I would suggest practicing with the engine OFF because moving parts on the engine pose an extreme danger and can be difficult to navigate.

Harbor Freight sells and automotive stethoscope that is inexpensive and easy to use for troubleshooting.