Don't know how I didn't realize that these plastic covers concealed the mounting screws... Thanks for revealing what should have been obvious. (In my 420's case, it was indeed just loose fasteners: on both sides, the top screw of the three needed significant retorquing.)
And it makes sense too--given a nearly 15 year old car. Between daily expansion and contraction (in and out of the garage) and variations in humidity, it seems virtually inevitable that some slackening will eventually occur. Once started, vibration, air pressure, and gravity conspired to exacerbate the problem.
Thanks again for pointing me toward this easy fix.
