Uneven load is the usual cause. Not much you can do about that. The main cause is the the gas dampers or springs in the m/c, usually 2 springs at top[ 2 dampers at bottom. when they wear the drum will move more, banging into the sides. The dampers will need replacing. watch the drum when it runs, you will probably see it go side to side. Most have a block of concrete inside fixed to the drum to help stabilize, but over time the dampers no longer can cope with the loading.
Sheepdog, March 2010
