I've just solved the problem on mine.
Pull machine out of unit space, turn off the power suply.
At rear of machine locate two screws near top of machine, they go through two plastic clips, one each side that hold the top cover to machine. Take these two screws out then from the front of machine, push down and back at the same time and slide the top towards the rear. The top will then lift off. Inside you will see a block of concrete (the balance weight), there are three bolts holding it down onto the drum housing top. These require tightening up, use a 16mm socket or spanner. Re-assemble the lid and replace the two screws. Problem solved. Turn the power back on. The bolts tend to work loose and the concrete counter weight flops about like a loose brick. Hope this helps.
Colin McKelvie, November 2007