Knocking noise

Hey all i have just changed me rear diff, prop shaft and half shafts on my disco to 110 4 splines and have just recently heard a single knocking sound when either slowing down, changing down and even putting it into reverse, just wondered if anyone could help point me in the right direction, all help is greatly appreciated. cheers

A Frame Ball Joint.

thanks chaps, but found out that the under lying problem was actually i was sold a dodgy diff, and that ended up causing all the annoying noises. but cheers anyways