My car has a humming...

My car has a humming noise when you put the car in reverse I took it to BMW Centurion and the technician told me that the discs are glazed and I should contact BMW SA which I did I was taken back to Fourways Technician Manager there told me its normal there is no way in hell that a high performance car over the value of R700 000 makes such a noise and is considered normal they did not even compare it with other cars of the same class