Hiya. the bottom arms shouldnt really have much movement but if the rubber bushes look really perished or peelin away around the edges it could be worth changing em. if the arms are pretty bad then u may get the steering wheel snatching as u hit bumps. and the anti roll bar should have no movement whatsoever in the bushes. thats more than likely ur knocking noise as happened to mine recently, only couple of quid for the bushes anyway.