[Northwood Cobra]

I've got a tremec 5 speed and a 9" limited slip with 3.50 gears, if I put the car in reverse and let the clutch out, as power engages, the rearend gives me a sound like a hammer hitting the case, it only does it once as the driveline starts to turn and then it sounds fine, it seems to be worse when I engage the cluch fairly quickly, is this normal?

Terry