I've got a 2006 Silverado and an odd noise recently started when I go over a rough spot. It sounds like something is oil canning. I can duplicate it if I rock the bed side to side hard and can actually feel it.
I checked all the bed bolts and those are tight. I can't find anything underneath that's loose. (Actually I found several things but none of them were the noise.)
My local mechanic says its the axle working in and out of the rear differential but not to worry about it. I jacked up the rear today and both rear wheels will shake if moved vertically but not a horizontal shake. It's a G80 rear on a 4x4 if that makes any difference.
It may very well be the axles but how is that "nothing to worry about"?