If you hear it from the rear of your car and it sounds like someone knocking on a door, it is your fuel system pressurizing.
Helps fuel ecomomy and blah blah blah. There was a thread about it a while back.
You say it sounds like the noise comes from the rear, and last time I checked, the pistons were in the front.
EDIT: Here's the thread:
http://www.camaro5.com/forums/showthread.php?t=26900
Read post #11, it's from the Fbodfather himself