I just checked again and I'm 99% certain that its the gearmotor assembly that's vibrating. I can put my finger on the assembly and feel the vibrations. How foolish would I be to disassemble this? It looks like once I remove the two mounting screws, there are only 3 or 4 small screws holding the gearmotor housing together. I bet one of the little gears in there is not meshing.