The head unit in my 2000 A6 just died , so I replaced it with an Alpine CD player, connected up to the half-Bose system with autoleads connectors.
With the engine running, I could hear a faint whining noise. Ejected the Coldplay CD but the noise persisted - at engine speed, a very faint echo of the noise you'd expect from an unsuppressed alternator. Not noticeable with music playing, although you can hear it in the background with talk radio.
I assume that the alternators on this age of vehicle have built-in suppressors, and this interference wasn't evident on the original unit when it was working.
One suggestion was to run an independent fused mains supply and earth direct from the battery, bypassing the loom. Any ideas whether this would work? Is this a known problem, and is there a known cure? I'm not sure that fitting an external suppressor to the alternator is going to be feasible.