sounds like pinion mesh is too tight, when you go to tighten it down check to make sure the motor holes aren't stripped out. Sometimes if you crank the screws down the holes strip, the cans aren't steel like they were with brushed motors. I've had good luck putting a dab of blue lock tight on the motor screws and tightening them down pretty good, obviously don't go gorilla grip or you'll strip the motor, but you can get it nice and tight.