I wonder is there any way to prevent this noise... Maybe micro servo and audio equipment just don't work together.
Electric guitars/amps are Hi-Z input and it's very easy for noise to couple into it. (example: AM radio, RF buzz, fluorescent lamps, etc)....
I suspect the micro servo motor (unshielded even!) butt up against the metal case of the amp is a sure fire way of transmitting the EMI/RF noise of the servo to your amp.
You can try "mechanical shielding", see faraday cage, and then grounding that metal case. But the fact that it's supposed to be butted up/pressed against your guitar amp (to be able to turn the knob) I don't know if that will still be effective.