my 08 Escalade had the same problem and I found the motor assembly loose. One bolt actually fell out and the other was barely in by a couple of threads. I took the necessary panels off and re-tightened the motor and no noises at all anymore. Easy fix.
One of my 2012's did that it was an alignment issue, the liftgate need to be slightly adjusted, once that was done the popping noise was gone, what caused my noise was the liftgate was being pulled slightly by the power latch to one side, if you would watch is close a few times you could see the whole liftgate being pulled to one direction, it would open and close alright just made that pop.
Grease the shims on both sides of the hatch. You can also loosen both of them, open and close the hatch a few times, then just lock them down where they seat. Make sure you put a bit of grease on them so they slide smoothly when the hatch closes.