Same issue here. Just purchased a '15 Silverado 4x4 5.3 crew cab. With around 400 miles I started to hear a noise over the radio. Has two distinct sounds. When accelerating, mostly at speeds 30-50mph or going up hills where drivetrain is under load, I get a metal on metal sounding "CLICK". Once the accelerator is released the truck makes a distinct hollow "PLUNK" noise as best I can describe it. Almost like a bouncing ball type of sound w/ a hollow metal feel to it.
I've had it to the dealer and got the same bulletin #99-04-20-002I. Basically states slop in the transmission and driveline and is completely normal. Nothing about this seems or sounds normal. Just traded in an '11 Silverado 5.3 crew cab that had to have the pistons and rings replaced due to an oil consumption issue with only 50K miles. Now I am regretting the decision already of purchasing another GM product. Going to be a long 6 years if this truly is normal operation.