we had this problem.  we followed the instructions in the manual to check the drain filter.  We didn't find anything.  The called the service man - he got down, removed the drain filter and shone a torch to the pump beyond, and found a rubber band had got past the drain filter, he pulled it out with pointed plyers.  He said it was a common problem, also hair clips get down there.  Anyway, problem is fixed. So try that.
                 rebecca
                
 July 2008