Hi Lee
Did you get to the bottom of the issue?
I've just had more or less a total rebuild suspension wise on my A6 Avant and the knocking is still there and driving me totally nuts! Been back to the garage and they've double checked it...nothing they can see.
Mine is when I'm about 30-40mph, over bumps etc. Rattles like hell. Sounds like rear passenger side. Garage thought it was more rear driver side but I drive it all the time and not sure I agree.