Has the drum cracked ?
Open the door and look at the back face of the drum.
Look carefully and you may see the face flex as the drum rotates and a crack or 2 may appear!.
The sheet metal will flex until it reaches a point where the tension is released and it bangs back into place.
My BG447 has just this problem and is 21 months old!
eremy Downing
May 2007