I've had an ongoing problem with the rear leaf spring making a loud thud when going diagonally into a driveway to keep from scraping. It's on an '04 Z06.
I believe it started after I had a fuel pump replaced. It's my understanding that they had to drop the rear end and one of the gas tanks (?) to replace it. After that I started getting the "thud".
I took it to the dealer and they said the spring pack was loose and they tightened it and charge me an hour of labor. The asshole service guy told me they repair anything in the drive train but don't tighten bolts. This pissed me off of course since I have the extended warranty and since it was to tighten something I believe they didn't torque right when doing the fuel pump replacement. So I paid the crappy service tech and went home. About a month later I got a check in the mail from Chevy agreeing that they should not have charged me. :