Just an update on how this story ended.
After further driving it became very obvious that something the catalytic converter closest to engine made the rattling sound. The porcelain(?) inside of the cat was being crushed into smaller and smaller parts and would eventually clog the whole exhaust pipe. This would be a bad thing if engine was running at high RPM’s.
An exhaust specialist used a simple detection method: Standing under the car (on lift) we looked at the Catalytic converter. It has two “chambers” where the mesh is located easily detectable from outside. He then simply tapped with his knuckle on the chambers and the difference in sound was obvious and identical to what to expect when tapping on a wall for finding cavities.
It was impossible within my timeframe to go for aftermarked. Re-used was not an option as this is a very rare car in Norway. Probably less than 50facelift (of approx. 4 million cars in country) exist due to stupid taxation of engine size and power
Unfortunately sourcing an original CAT from audi was close to impossible. The stealership claimed that there was no OEM NS cat anywhere in Europe and that a new one had to be made with a very long lead time
. A local highly skilled Audi tuning specialist managed to source up a cat from Germany in two weeks and I had it changed. This was expensive, but I survived and can continue to enjoy the S8.
So, there you have the full story. The CAT was the culprit. Thanks to all that helped me with advice in time of despair.
Kind Regards
Yggdrasil