Washing Machine tripping RCD on mains?

had a problem with washing machine not being able to complete a fast spin (eg 1200-1400) a couple of times. We went on holiday for a week and I left the machine plugged in but switched off at the socket, the machine was dry and empty.
When we got back i switched the socket back on and the RCD tripped immediately. It wouldn't come back on until machine was unplugged. We tried a different appliance in the socket and this worked fine.
I have had an electrician friend inspect and he says there is a neutral to earth fault on the washing machine, or earth leakage but washing machines are not his area of expertise.
Could anyone suggest what the problem may be? and how we can fix it?