I get lots of flappy body noise when rear wheels traverse over poor tarmac. (BTW quite normal for South Bucks).
I discovered the reason after removing rear wheel under arch:
Part B (the under arch cover in photo below), is a flappy soft fiber construction of two pieces stapled together. It is the only means for giving rigidity to Part A (i.e the plastic piece of body trim which wraps from the rear bumper).

Because the whole of the bottom edge of undercover is so flimsey and only fixed by one screw, the weighty bottom area of undercover distorts over small bumps and Part B flaps with it.
I only checked the nearside rear wheel so far, since most of the noise was coming from this area. It can be checked by hand without removing rear wheel. Simply grasp the bottom inner corner of the undercover A and feel it's freedom to move. The body trim B will move with it.

Any other Arteon's with this issue?

Click image for larger version. 

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