My car was showing same fault, reset, it would just come back after a short time, but was starting and running ok, it rattled on start now & then and occasionally under acceleration, advice I was given was that if it rattled on every start, then the chains/guides/tensioners etc all needed replacing soonest.
not sure what other symptoms you might have, there’s a lot of sound insulation in the A6, so it’s difficult to hear the engine on start with the doors & windows shut.
But either way, good luck.