I "sounds" like your stripping the gears? If the pinion and spur aren't stripped and the motor is bolted tight against the motor plate so it can't move, then your problem is likely elsewhere. The slipper clutch may be too loose causing the motor to rev-up faster than the car is accelerating. This confuses some people because we're used to electric motors winding up as the vehicle speeds up. This is especially true if you punch the throttle from a dead stop and you get little to no tire spin (burnouts) off the line. Simply adjust the slipper adjustment nut on the right side of the vehicle until you have minimal slip. Note that some slip is good as it relives excess force from the gears so they don't strip. Another possibilty is that the ball differential is adjusted incorrectly and is too loose. Even with light tension you will still see a "differential effect" when the vehicle is running. And the vehicle will drive reasonably well with a loose differential, however, loose ball diffs cause a harsh "barking" sound when they slip. It's most noticeable when the car is jumping and lands while you still have the throttle pulled. The sudden bite of traction upon landing after the motor was revved in mid air causes the wheels to match the ground speed while the motor and transmission still have built up inertia. The difference in speed causes the differential balls to slip against the out drives and creates that "bark" sound. To adjust this, you need to remove the right side dog bone so you can access the allen screw inside the outdrive yoke. You must hold the opposite wheel and spur gear while you adjust this, otherwise the opposite tire will turn backwards. Make small adjustments, it doesn't take much to tighten it. Too much tension and the diff will feel stiff and act more like a posi-trac rear end. You didn't mention if the sound is continuous or only during a hard acceleration but, there may also be debris inside the transmission which is running through the 3 internal gears. Open up the case, clean the gears and apply a light dab of grease on each gear. Don't add too much or the grease will cause excessive rolling resistance.