Stepper does not move smoothly I have a problem with the y movement of my machine.

When i select y+1 in repetier, it moves, but with a very strange noise.

I have a video to show the problem here

I have:

Set the voltage of the stepper drivers to 0.6V

Set the voltage of the power supply to 12V

Tightened the belts (but not too tight!)

Does anyone know what this is? Is it a problem with my belts or are my stepper motors too weak?
Fixed. One of the two was movign in the wrong direction. Problem solved!