It's a ground issue on the right side and you're getting feedback from the ungrounded headlight. To test, clamp a battery jumper cable to a clean ground on the engine or neg side of the battery and clamp the other end to a clean spot on the back side of the light housing and see if the issue goes away...if it does then the rusty fenders are no longer giving you a clean path to ground. Run a new ground wire from the frame to a new clean spot on the fender or to the back of the light.
Mike B
