Make sure your gears are all aligned and you have the spacers installed where needed to prevent sideway slop back and forth. Check the motor alignment . It can quite easily be skewed. The metal towers are not that stiff. A light twist might be required to get the pinion meshing properly with the first gear. Once you are happy with everything, put some mild abasive in the gear train, and run them for 20 minutes. Wash out all the abrasive cleaner and lube them. You should hear quite an improvement. In some cases I have had to solder the brass bushings into the tower. to stop them from moving around. If you get a little slop in the bushings it can be enough to get the gear off of being true and you get the grinder action again.