Odd sound when backing up and wheels turned to the right

Started a couple days ago.

Pull up and go into reverse, cut steering wheel to the right, start to back up and just as the truck starts to move theres a loud pop.

Happened again today, same situation.

Truck tracks straight, no pull.
No vibrations at any speed.
No odd sounds other than the one loud POP as described.

Any ideas?

 

There's a thread on popping somewhere on here. For some reason I want to say rubbing on the skid plate come to mind but I would have to dig for it again.