I believe it's definitely caused by engine vibration. My buddy doesn't have his motor mounts just metal to metal. He uses a combination of leather belts and rubber from a thick inner tube. I've told him that is the cause of his vibration he feels while riding his motorbike.
technically, no. the cause is the unbalanced crank.
a strong connection of the cases to the bike frame lets the frame absorb the vibration. some of it anyway. maybe enough to keep from driving you crazy.
me, I'm a perfectionist. I don't mind working a couple hours to get rid of the vibration by balancing the crank.
OK, but if the clip is rotating on the needle because of engine vibrations. But the needle has notches for clip placement it must have ground it smooth then skipped to notch next to it while throttling up and down.