Originally Posted by m4a1mustang

Is this your first car?

Drive line "clunk" on RWD cars is pretty normal... There's always some play between the components. When you apply power the drive train loads (from front to rear, obviously), and as it does you may hear a clunk from the rear end... The same thing happens if you pull out of the throttle rather quickly in a high gear.

This is the reason why, when at the drag strip, experienced drivers tell you to "load" the drive train up when you stage so you don't shock the rear end on launch.