No, not a common failure point, but that's normally felt as a slushy steering feeling rather than a knock but this might also explain some of the wobbly steering feeling ..
Stupid point, your tyres are all at correct pressures, and don't have a rock/stone embedded in the tread causing a tapping???
This is what I'd do to try to get rid of it .. It's my experience that knocks and slushiness don't get better until something fails, so try to be as thorough as you can be. You really need to check all the joints in the suspension and driveline, there are loads!!
I'd be tempted to lift it onto axle stands, start at the rear and take the wheels off to properly check/grease the wheel bearings. Whilst there check the damper bushes, you have to work the suspension to check them properly, so carefully use a jack on each side .. the damper joints normally feel OK when the vehicle's just sat, so unload them to feel for play.
You've done the radius arms, so now check the A-frame joints, use a decent sized pry-bar, you won't move much by hand.
300 Tdi Disco, check the prop rubber donut at the diff, if it has any splits change it, you might get a surprise at what a small outside split disguises! Pry bar to the front UJ, hold one flange solid and try to turn the prop, any play or a tap if it moves means you need a new UJ.
then do the same at the front.
Habe someone turn the steering fully right and left while you rotate a wheel/hub .. if there's knocking, or tapping especially towards the end points of rotation, or you can feel a regular vibration it might be the CV joint .. indeed it might be the cv joint both sides, so don't find a bad one, fix it then find the second one needs doing too .. Same time as this check the swivel seals for stiction and grease/oil smearing across the balls and the swivel bearings for play, there shouldn't be any.
Check the the front UJ's and prop same as for the rear. then move to the steering bars, trackrod, draglink and panhard rod, same as other bushes, use a pry bar for play. All the joints should move, but should move in a controlled manner, they shouldn't tap or knock when they move, but instead they should be free to move but not be very tight, and definitely not a loose movement.
Have fun ...
Don't forget, if you find something wrong, it doesn't mean that's the only fault .. In theory if you find a fault and fix it, you'd then do a road test to see if that's the source of the problem 'only fix one thing at a time' but that might take ages.
At this point I'd suggest you need to check everything, make a list and go back to fix any issues, there might be more than one, before road testing.
