Airsoft Motor Problem PLEASE HELP!!?

Hi i have a airsoft motor problem. I have a thompson airsoft gun, with the wiring in the stock. Before I connect the stock wiring with the wire that comes out of the gearbox, when i plug in my battery to shoot the gun, it works fine. BUT, after i plug in the stock wiring and connect the battery, the motor makes a grinding noise and it shoots about half as slower. It's not the fuse, or the stock wiring I checked both. It's not the battery cause i have a 9.6 it has to be the motor/gearbox, but I don't know what to do. help?