Pads rattling in the carriers? Ie worn carrier rather than anything peculiar to the pads themselves.

Had a similar noise on FL D2 S8 when the cats started breaking up internally.

Had that too. Open the door, stick your head out and rev the engine. Rattling cats should be obvious if that's the problem

Very obvious indeed. I bought my current S8 with a rattling cat. Though by the time I finally got round to replacing it the rattle actually stopped - nothing left inside!

Not rattling cats.

On brake carriers there was some indentations where the pads rest so I kissed them with grinder to smooth them out. the springs on top of the rear pads keep them pressed tight. This moves pad toward center a fraction. I also switches carriers left right because one of the guide pins are usually slightly sloppier than the other because of the direction of rotation.. I have done this same treatment to all my Audis and friends cars also. Great success with no down side on any other cars. Probably 20+ cars in 10 or 12 years. In fact more even pad wear. Had rattle before and after carrier swap/treatment

I When I stripped the entire trunk of everything and even removed rear bumper. I rode in the back seat leaning out the window. The sound appeared to come from the right rear wheel well (although I didn't actually lean out the drivers side...hmmm bet it sounds horrible on that side also).

I am down to the rubber surround on the center driveshaft. It is probably deteriorated to the point that it allows contact. Funny that I have some vibrations on highway but nothing major. I also live in area that I seldom drive faster than 50-55. Mostly country roads without long stretches of smooth pavement. I always blamed vibrations on low profile tires and road surfaces. I think the sound must echo down the tunnel created by heat shielding. My street is the acid test of vibrations. When I have the windows down and hit bumps people turn and look. Sounds like a garbage truck.

I'll be doing exploratory disassembly when I get the rear transmission output seals and get some picks when I find root cause.

thanks for all suggestions.

Something that might help locate/eliminate sources: Attach a cheap GoPro type camera underneath, pointing at anything you suspect might be the cause.

You need to get it on one of those 4 post road simulator shaking lifts, not sure if or where are they available in UK but some countries use them for MOT equivalent tests, I think one of them is Germany.