First post here...I purchased my 2014 GMC Sierra SLT in October 2014 and have had this same issue. The service advisor at my dealer went for a ride with me and said he had another customer had the same claims of the rear end noise. After reproducing the noise for him he thought it was the exhaust system...I thought he was wrong as it sounded like gears grinding at times. They replaced the entire exhaust system on my truck and it has been noise-free ever since. I talked to the service advisor today and after replacing the exhaust system on the other customer's truck, he said the guy told him that he no longer has the rear end noise either.
To those of you still experiencing this issue, please have your dealer check the Julian date on your muffler to see if your vehicle is covered by the TSB.
I would have sworn the noise was in the drive line or rear end and several buddies said the same thing. I didn't believe the dealer when he told me it was his opinion that it may be the exhaust system...turns out he was right in my case.
