Oh dear. I feel for you as I've just had this on mine. Seems to be one of many common faults on Audi's across a number of years. What appears to happen is one of the two motors of the central locking system in that particular door go faulty. You will need to remove the door card, get inside the door and pull the central locking unit out from that door (white). Once you've disconnected it and all of it's cables you need to take it apart, find the motors, pull them apart and blow out all of the carbon inside. Then re-assemble them and clean the contact with WD40 before reseating them and reassembling the door lock.
The alternative is to buy a new door lock (~£100). I should point out that many people have come across similar problems and they have not always had the same solution to their problems. But for mine this happened with both the drivers and rear passenger doors.
I did have a link for removal of the door card and lock somewhere - if you Google it it should come up.
Good luck
