Mine used to grind too. I opened up the gearbox and motor and found it well lubed. The problem is the cheap nylon gears were not meshing when under a load. I loosened the motor mount screws a little then pressed the motor end housing away from the gearcase. It will move a fraction of an inch, just enough to press the gears together a little better. Hold it in place and tighten the screws real good. No more grinding/jumping.
