I've had my truck for about a month (1400 miles on it). I heard a few pops coming from the rear area of the roof a few days after buying the Sierra SLT.
I took it to the dealer, they said it could be coming from the third brake light. They had the truck for over a week, said they took off the light and replaced it.
Popping noise still happens from time to time, even when parked. Sounds like something falling from a tree hits the roof.
Looking forward to if anyone finds a solution to this.