Chattering motor

[WebDog]

I've got a chattering motor. :) It seems to run smooth when the car is
not attached. Sometimes it chatters without the car attached. I've
tried two speed controls, motors, batteries, and crystals. I wonder if
I put the brushes back in upside down? I put the braided wire on the
top and the spring below towards the motor can. I'm pretty stumped.

Thanks,

WebDog