borat52

I've had my imprezza for 2 years now and always had this problem.
When in 1st or 2nd gear when cruising if i suddenly take my foot of the accelerator theres a horrible knocking noise and the car jerks a little. This only happens when the clutch is engaged (ie if i quickly dissengage the clutch its smooth). Its really beginning to annoy me now. Had a new clutch fitted when the car arrived and thought it might 'bed in' but the problem has always been present. What could it be? Knackered box?diffs? I'm clueless when it comes to the drivetrain but surely there can't be that many things that could cause this problem.
thanks
will