Had this exact issue during the rain a couple of weekends ago.
2002 A6 Avant. Needless to say the car is somewhat out of order, I have
the car located at the Princeton Audi dealership, delivered via
trailer. They have had the car 3 days thus far, have other cars with
the same issue that they are fixing . . .
Wish I knew about this before the rain . . .
Hope you just have damp carpets chap !