I tried to find any thread on this...

I have a 2008 40th Anniversary RX8 with ~2k miles on it. I noticed that when I put it into reverse, only after it has been sitting for a few days, that when I start to back up, I hear a thud in the rear of the car. It is almost like something is breaking loose. Today it happened again and my 5 y/o kid in back really noticed if as well. Also a noise continued from under the rear center of the car heading forward for a few hundred feet. The car seems to drive and shift fine.

I looked under the car and don't see the typical drive shaft but some boxes in the back where a conventional differential would be. I am a bit unfamiliar with the RX8 (I just got it used) and am also trying to find any schematics that explain the drive train system. Anything listed online?

Any thoughts on the "thud" heading into reverse only after a few days rest?

Thx!

After the car sits for a few days the Brake ABS system needs to reset itself, and you putting it in reverse and hearing that thug once the car moves is exactly that. It isn't anything damaging, you'll be fine.

yup, perfectly normal. :)