Berlingo Van 2009 1.4 Petrol Steering rack knock

I have a knocking from within my power steering rack. It started about 3 months ago, when I noticed a creaking groaning sound, so I had new tyres fitted and the tracking done. The tracking needed 3 adjustments to get right - but even then it's not spot on - and it wallows. I had my mechanic check things, and he determined the rack as being the cause of the knocking. For my peace of mind he changed both lower ball joints - resulting in less noise/vibration and a softer ride. He determined that there is no play in the rods (joints - both sides both ends). SYMPTOMS - When driving, although car steers pretty straight, it has a tendency to wallow/wander and veer left or right depending on road condition. It is very camber sensitive too. This can be exaggerated when applying the brakes when coming to a stop - sometimes the pull is alarming - can be either to left or right. The n/s knocks when going over bumps/pot holes - and kicks back to the rack, where I feel the effect in my steering column and at my feet. The right side seems fine. On standstill - engine on - rotating the steering hard right and rocking the steering wheel slightly induces what seems to be extra movement and the knocking - doing the same in the opposite direction is fine. I also have what seems to be a 'torqueing' sensation/movement at odd times, as if something has been pushed out of position and then corrects after a few seconds, depending on manoeuvres made during journeys - not apparent before. My question is this. Why would the rack knock internally and does this mean the rack is shot. My mechanic, with my help has checked all possible causes, including loose rack bolts, engine mounts, strut mounts etc. The knock is quite definite and can be felt when manhandling the n/s/f rod and moving/rocking the steering hard right. Any ideas about this peculiarity? Thanks.