Golfer 17

My A4 has recently developed a very unpleasant smell like damp carpets, particularly evident when opening the door after the car has been left got some time. It does nor appear to come from the aircon. Took it to main dealer who said it is not damp related in that all carpets are dry. They don't know cause and didn't seem particularly desperate to find it. Only recommendation was to hang a fragrance dispenser in the car. Has anyone any helpful suggestions?