2003 Sierra 5.3 4wd 139k miles
 

Hi everyone,
I have a light erratic drumming sound from possibly the right rear. It is random at all speeds but gets a little louder above 40mph. It's not in sync with the speed. Rear brakes, calipers and rotors replaced about 3k ago. I checked torque on leaf spring u bolts, lug nuts and removed lower bolt on the shock to check it, its ok too. Differential oil replaced about 3k ago, no metal in it, put in exact manufacturer oil and filled exactly.

Any help is appreciated!