This is a simple solution to an aftermarket stereo faceplate that is loose and vibrates.  Just find some very thin foam, and cut it out to fit the back of the faceplate, then smoosh it in between the faceplate and the stereo headunit.  Mine required no adhesive.  It’s rather tight snapping the faceplate on afterward, but that’s the idea.  If you regularly take the faceplate off your headunit, you might want to find some extra-thin foam, or try using less of it than I did.