If the axis is creeping and the ball screws are 'good', then the motor is turning.
If the motor is turning and it is a stepper motor, the driver must be telling it to do so.
If the driver is turning the motor, it must be getting step commands.
The step commands could be noise on the signal lines, or they could be real step commands from Mach.
Check:
Get some LEDs and some suitable resistors and hang them on the driver outputs.
Can you see the LEDS changing from on to off and back?
Kill the Mach program. Are you still getting creep? If so, noise somewhere in the cabling is to blame.
If you are only getting creep after starting Mach, but issuing a command stops the creep, then maybe your version of Mach is outputting step commands very very slowly. Hang a LED and resistor across each of the the Step and Dir lines: can you see any pulses?
Note: a few of us have seen Mach continue to issue jog pulses after we have taken our fnger off the jog key, and issuing another command stops the movement. However, such events are rare. If it is happening to you reliably ... ummmm ... problem.
Cheers