Been trying to hurt down this knocking noise in the back end of my 2008 Clubman for a LONG time. Today I decided to stick my hand into the wheel, grab the rear sway bar and noticed it has some side to side play in it. The ridges in the sway bar seem to be knocking up against the sides of the bushing housing causing this annoyance.

So I guess it's time to fork out some cash and replace those bushings. Hopefully this may help someone else trying to hunt down any knocking noise in the back end of their Mini.