The setup is in a word - crap. No mechanic here but let me attempt to explain. The sissy bar ends slide into the strut hole/groove (which is normally hidden with a "filler" - barely visible in the posted pic above). The part of the sissy bar that goes in the groove looks exactly like the filler. Anyhow, when you take out the "filler" you'll see it has two half-circle notches cut into the sides (front and back). Those are for "balls" that are embedded in the strut that get pushed via another set of screws accessed vertically under the strut. I.e. when you screw the vertical hex, as it moves upward it pushes the ball horizontally into the notch in the filler. And that's how the sissy bar or the filler is kept in place.
Now, beyond super complicated, that bad part is the material. The strut is aluminum. The vertical screws are also aluminum. I have gone through 3 sets of screw and stripped/cross threaded them every time. The metal is so soft its unworkable. The service manual supposed says 3 foot/lb of torque which I understand it to be like a fart. I intend to fix this using steel components - a helical and then steel screws.
Back to your issue, i think the rattle is happening because the balls aren't fully seated in the groove, meaning the vertical screws are not all the way in. But note my caution. If you try to tighten it more you could end up with a screw stripped and embedded in the strut. Like my left side. Now I have to take it to strokers so they can drill it out.
