Does the mirror wiggle around when you can hear the rattle. If so, the mirror need to be tightened. The rubber boot between the mirror and the body can be pulled loose from the body and the mirror.
Pull the rubber boot loose and move it aside and tip the mirror either forward or backward and you'll see the screw holding the mirror to the bike, make sure they are tight.
It take an effort to tip the mirror, but you aren't going to brake it.
From the Northwest Corner.
Ron