I've just had the MOT done, and still not picked the car up yet as I'm waiting for tyres - I've been struggling along in my 330 convertible instead ;-)
Anyway, I've had a similar knocking from the NS front suspension, and figured if it didn't fail the MOT I'd at least ask them to take a look while it was up on the ramp.
Well, it failed for the upper wishbone balljoint, so the whole arm was replaced on Friday (I think they sourced it for £79 plus fitting).
On Saturday I was on my way to view a gorgeous little Smart Brabus Roadster for a friend, and thought I'd pop into the garage to see the progress and get a look underneath the car.
That's when I learned that there had been so much play, that the inside sidewall of the tyre has been rubbing on the arm, and in a small strip it was worn down to the canvas!
Now, my knocking noise wasn't horrendous, just a little annoying - it was one of those 'I'll need to think about that soon' problems. I really was shocked to see the state of the tyre - so please park up with full lock and check your inner sidewall.