Bruddah, your spur gear is TOAST. You didn't SEE that?! Get a new spur gear, set the mesh properly, and you should be fine. I didn't even look at the last two links. That's your problem as far as the noise is concerned.
But you mentioned that you felt the car didn't get up to speed as you expected? What was the KV rating of the motor you replaced, and the KV rating of the one you're running now?