I have a 2003 Sierra with close to 10,000 miles on it. Every time it shifts from 2nd to 3rd, there is a clunking noise. It seems to be coming from the transmission or diff. It sometimes rather than being a clunk, sounds like something slipping when it goes into gear. Anyone else have the same problem or suggestions on what it might be?

Automatic or 5 speed? Either way, at least it's under warranty.

Automatic

I have the same problem with a 2003 supercab 4x4. There is a TSB about the very problem. Mine just turned 10000. I have not taken it to the dealer yet but, from the posts it sounds like you have to live with it.

Yeah that's what I figured, but I was hoping maybe someone had a solution or fix to the problem.

my truck was making that sound i brought it to the dealer and they said is was my u-joints. when gm made this truck they used greaseless fitings on the u-joints so i went to the auto parts store bought some u-joints with grease fittings on they installed them and the noise stopped