It’s impossible to be absolutely sure but from what you describe (having ruled out vibration) it is more than likely electrical noise from the VFD being picked up from the limit switches / inputs wiring (including e-stop, touch probe, etc.).
My opinion which is basically the same as others have said in your earlier thread…
If you haven’t done so already, create a single point Earth at the controller, connect this point to the incoming Earth and connect the shields of all inputs cables to this one point (leaving the other ends of the shields unconnected. Re-connect 0.1uF ceramic capacitors between each used input to the BOB and the single Earth point.
The wire from the VFD to the spindle motor also needs to be shielded and again the shield is connected to the single point Earth and left unconnected at the other end.
If found necessary, re-route the wire from the VFD to the spindle motor separately to the other cables. Check that you (or the Chinese) have not created any GND loops or Earth loops in any of the wiring (these make wonderful antennas).
Hope this helps,
Tweakie.