First things first, have you checked the motor or truck for binding with the motor removed? Are you certain the gear mesh is correctly set? I know these may be insulting if you know what you are doing, but we have to be sure before we continue. It is not recommended to rev a motor to high RPMs without a load, but you should try and run it lightly out of the truck to determine if the sound/binding is coming from the truck or the motor first and go from there. If the motor is simply cogging and not turning it will certainly heat up to dangerous levels very quickly.