Hi,
My washing machine used to do this. The reason was; The pipe that connects the washing machine to the outlet on your sink/drainage pipe is not positioned correctly. The pipe from wash machine MUST be in a U shape 8-12 inches above the point where it connects to the drain pipe to the outside drainage system.
Basically the water MUST NOT be able to drain from your sink/drain pipe back into the washing machine. Example when you are doing washing up of dishes etc. Any water that drains down your kitchen sink can drain into your washing machine if the pipe is not positioned as I've tried to explain. Any problems send me your email address to 07927 161202 and I will explaoin it with a diagram.
Paul Yak, April 2009