I started having this noise happen around 3,000 miles (5.3, Z71, standard box). It would always occur between 5-15 mph. It was kind of a rattling noise coming from the rear end, sounding something like a bad U-joint or a posi-traction noise like something was loose. After 15mph the sound would go away. I made an appointment with dealership. He stated that no one had been in for servicing with that problem and made that quote all service writers have, "if we can't duplicate it, we can't fix it". My rear end noise went away about 200 miles ago, so I never took it in. Curious what other dealerships are doing to resolve the issue.