when u choose a teeth set u have to use the right combination, make sure that u put in the right big gear as well. It could be grinding cuz u didnt put the new big gear in too. Or if u didnt put the teeth on the motor hard enough and it stix out to far it will rub on the plastic casing, make sure that u push the teeth as far as they will go on the motors shaft. look inside when u have evrything together, see if everything is meshing right before u put the see through cover on and give it a little gas (or electricity
)