Originally Posted by Ruffy

I bet it is from the end of your pinion rubbing against the gear cover.
Try this... take the gear cover off, try and see if sound goes away and you have free play. If so, forget about getting a full mesh to your spur gear and mount your pinion on the motor a little bit more in, like 3/4 engagement or 1/2 engagement to your spur and then mount your cover again. Check and see once again.