I found that my 2007 and 2008 Denali's both did it when backing up and turning sharply to the left. I thought it was clearance from the mud flaps for the longest time. Finally after getting pissed off enough, I removed the mud flaps and inspected the fender.
I found the driverside wasn't the same as the passenger side from the factory in their stamping process. The passenger side had a nice 90 degree angle and the driver side was slightly less than that. I got out a deadblow hammer and whacked it back to match the quiet side. Installed flaps and has been quiet since. FWIW