When I took mine apart, I found a ball and socket affair that hold the adjustable mirror. Plastic, of course. The socket no longer holds the ball (mirror part) tightly, so it rattles. And the only fix was to replace that whole assembly.
BTW, the mirror is partially glued to a plate which has the ball on the back, and you can Google how to get all that apart.
The cost of the parts led me to live with the wobble so far...but you have inspired me to fix it.
Best of luck!
2014 Itasca 27n