I knew this was something I was going to have to describe better. It sounds like something shifting. It isn't a squeak, or a thumping but kind of between the two. My dad thought it might have something to do with the universal joint or the positraction.
Probably definitely want to have the rear leaf springs checked out - there have been some bolt problems back there I understand. Also, had a creaking type noise in the rear of my C5 Z06 and it turned out to be, and I quote:
"35872 Excessive Clearance Warr ***** 2.7 OL PLus OP ALAIN *****
Multiple noise isolation/diagnosis R&R right side halfshaft and replace inner CV/Tripod joint. Muliple road tests (16 miles total) 2 noises observed: 1 Slow turns: isolate to differential, 2nd (after 1st repair) to halfshaft, raise on lift verify component torque rear suspension components: tighten rear cradle/crossmember (2nd noise/click type noise with load changes), road test figure 8's, 4 miles suspend fluid. Drain/flush/refill differential. Road test figure 8's 4 miles, 1st noise (grind on slow turns) resolved, 2nd noise click/clank on load changes (compare to like vehicle: this vehicle much more pronounced.road test with Alain, SI search, advised to continue isolation) raise on lift, evaluate rear drivfe components right side inner tripod joint has more play then left (noise frrom right side axle area) upon disassembly of R/side inner joint observed 2 colors of grease greenish (typical) and grayish (metal wear) retest after repair OK: typical driveline engagement with manual transmission normal operations"
The best part was when they tried to charge me to flush the rear differential ($160) for the noise while under warranty - and it wouldn't have solved the problem. Let's just say that Chevy survey got a few choice words returned on it.