Hi,
two mechanisms for noise, conducted and radiated.
Conducted noise is often overlooked. It is where the 'hash' impressed on an electrical supply by a device, in your case a spindle,
which then gets conducted to other devices on the same power supply, a servo drive for instance. As an experiment
run your spindle from an extension lead plugged in as far removed (electrically) from the supply for your PC and servos as you
can.
The other mechanism is radiated where the hash generated by a device is radiated to other devices and circuits by
electromagnetic radiation, ie radio waves. There are numerous techniques for reducing the sensitivity of a circuit to
radiated interference, a search of this forum alone will result in hundreds of ideas. Still the best bet is to try to reduce
the hash generated by the spindle/VFD. Line reactors are not popular with hobbyists, you have to pay for them, shock
horror but they are effective.
Craig