Hi there, had a similar problem with mine. I ending up buying a complete motor and linkage set, because two out of the three spindles had dropped due to the circlips holding them missing. This was causing part of the mechanism to be in contact with the windscreen. Also when I removed the old motor, I notice that it wasn't fitted correctly, there is a rubber grommet on the back of the motor which should be pushed onto the metal bolt sticking out from the bulkhead. This supports the mechanism. This is what was causing the awful screeching noise. Since I replace everything it now works perfectly. Happy days!!!
Hope this helps you with your problem.