I'm told that it may be "excessive backlash in the differential". The tech told me that some backlash is common in Zs, but that mine is particularly loud. They're going to re-torque the rear axles to be sure that's not the problem. When I pick up the car later I'll give another update if it's any better.

What is backlash in the differential though?

 

good question anyone

 

http://en.wikipedia.org/wiki/Backlash_%28engineering%29

To summarize, the sound is the meshing of the gears as they make contact and begin to turn the axles. If there's too much space (backlash) between the gears then the sound becomes louder and can lead to premature wear on the parts.

I didn't have the key for my wheel locks in the car so they couldn't pull the wheel to tighten the nut on the axle. I'll hopefully get that adjusted soon and will be able to let you guys know if it helps at all.