I was helping (well trying) to help Geoffroy over the weekend with his "noise" issue. His current problem is that when he runs the spindle all the steppers start making random moves. It is these rapid random stepper moves that is the noise he is hearing.
We started looking into the shielding of the spindle cable and he managed to improved it, but no change to the random stepper movements. He then removed the VFD unit and all VFD wiring from his control cabinet -still the same issue. So it does not look like it is caused by the cable radiating noise into the BOB.
The last thing we tried was to disconnect the parallel port cable from the BOB. This did seem to stop the steppers from moving when the VFD was run.
The reason I suggested he disconnected his slaved X axis motors was when the spindle was run these motors were sometimes going in opposite directions causing his gantry to twist. Removing them kept his machine safe, but did lower the "noise" from the steppers.
It looks like the BOB is seeing noise on the signals from the PC when the spindle is run, which is then being used to send random STEP and DIR pulses to the stepper drivers. This could be caused by an earth loop between PC and control cabinet or by a poor 0v connection between the two. More testing will be required to find out which.
Hope that clears up the meaning of the "noise", any suggestions welcome as to how to solve this one.