The motor is not bad. The gearbox is assembled correctly in my classic army m4. It keeps on blowing fuses and when it runs it whines really loud. The motor works fine in my other m4 so I need help.
whats the voltage of your battery? it may be too high, or the motor is using too much power too fast. the motor may also be too high speed. something is too much
It most be that your motor us not making contact with the spur gear. The spur gear is the gear that is supposed to connect with the motor but when the motor height is not adjusted properly, makes a whining sound. Look on the bottom of your M4 Pistol grip and adjust the motor height by tightening or releasing the screw. If it's not that, have you upgraded your gun recently? You must have a balance of gearbox parts for a gearbox to shoot a bb.