I too have had a clunk noise in the rear end of my truck when slowing for a corner and then slightly speeding up. It’s done that since new. The dealer where I bought it also told me it was normal. I now have 100k miles on it and I hear a crunching sound from the rear going over a slight dip in the roadway. I’m going to check the springs and ubolts for torque or replacement.
What you might be hearing is the infamous GM truck slip yoke noise.
The fix is to remove the drive shaft from the transmission and lube the splines with Green Grease.
Tester
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Wow, you are tolerant to live with a noise for 11 years!
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My 2005 does it too. The grease only works short term, in my experience. I didn’t use the “green grease”. Not sure if it would last longer. Some of the clunk, in my case, I think is just due to general drivetrain slack. Mine is older, with the 4 speed.
I think the above comments are likely correct, the source of the sound is the driveshaft slip joint. I wouldn’t expect there to be much of a problem with the rear leaf springs and u-bolts on a 2011. If lubing the driveshaft with a synthetic grease doesn’t do the trick, next in line
- driveshaft u-joints
- differential gear oil needs refreshing, esp if it is a limited slip , pos-itrak, etc design. These often require an slipperiness-modifier additive. Check the owner’s manual.