Interesting stuff, especially that the same problem exists on US cars. I wrote to my dealer a couple of weeks ago and said there were three other owners on this forum with the same issue - looks like the problem is quite widespread.
I'm guessing the problem differs in degree from car to car. My car has been very constant in level of noise. It's certainly low-speed only and most noticeable when a wheel drops into a dup, rather than hitting a rise. Strangely, bigger hits seem not to provoke the knocking.
I get the sound from all four wheels, but the rear left is still most prominent, by some margin. This is more than loud enough for you to think you have a lose rear exhaust box. Overall, the noise is annoying in town, as you're very aware of it and it just takes away a big chunk of refinement in what is otherwise an impressively refined car.
Got a dealer visit booked next week, with the head techie lined up to come out for a drive. I can't believe that BMW are not aware of this issue now.