Peter,
This to me looks like the broken wire or connector on one motor cable.
By swapping them, you move cable or connector and the electrical connection is (temporarily ) established.
The stepper motor always rattles when one of the phases is missing or it's intermittent.
Power supply (low) voltage is another possibility, however unlikely in my opinion, given that motors actually work (when swapped).
Last edited by bojan; 19-04-2011 at 08:25 AM.
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