Beyondillusion

Help-I have an'04 Allroad. The backup warning works intermitinly. The initial beep always initially goes on when put in R. Most of the time it gives no further warning. But now and then it starts working again. Dealer said it was fine but I doubt if they backed the car up. They probably heard the initial beep and stopped tere. I know this is probably a common problem but I can't find thread on the the forum.