I bought my 2015 Denali Crew cab just two weeks ago and mine is doing the very same - one metal on metal pop when I first accelerate and then again when I brake or decelerate. Dealer swapped the drive shaft and did not correct it - they too called GM and opened a "ticket" to see if the know of a solution. May call them and schedule a time to swap the leaf springs based on feedback here.
I GM monitoring this site? Surely they have run into this somewhere and have found the cause hence have a solution ... fix by trial and error has to be costly and frustrating for customers and jeopardize their loyalty.