Highly recommend you check all mechanical connections first. If the motor is 'whizzing' it could be something as simple as a loose coupling.

If you do get to the stage of replacing the motor, you would probably be better off changing the motor and maybe the driver. Without knowing what driver you have, and how it is wired, then it is difficult to advise you properly. 6 and 8 wire motors can be wired in different configurations and I would GUESS that you have a fairly common bipolar connection where the centre taps of the motor coils are ignored.

As Magicniner says - more details please !