My Focus seems to have aquired a knock on the steering. I first noticed it when parking in a tight spot and having the steering on full right-hand lock and it knocked when it reached the 'stop'. Since then, I've noticed that the steering isn't as precise as it used to be and on checking the tyres they had worn on the inside edge. I got the tracking checked but it was apparently fine, but the vagueness is still there and now it will knock when the wheels are straight ahead when you shake the wheel side-to-side - more noticeable with the engine off. I can't find any play in either the rack, the track rod ends or anything else on the front end, so am at a loss to what it can be. The only thing I can think of is a fault in the rack itself, but with there being no obvious 'play' I can't be sure. Can anyone give any suggestions what it might be?