The rear view mirror on my Hyundai Elantra Touring would vibrate and rattle when the car was moving. It was starting to drive me crazy, so I took it off and came up with a simple fix. The problem is the spring that holds the mirror to the window mount. It is a poor design, and it tends to allow a vibration and make a rattling noise. The instructions below are what worked for me, and I hope they work for you.
1. Remove the rear view mirror from the mount by pulling it down toward the dashboard. It will give way quickly, and I ended up slamming my mirror on the dashboard by mistake. Hold on tight and be careful.
2. Apply a loop of electrical tape to the edge of the rear view mirror mounting area. Try to cover about two thirds of the springs that stick out the sides.
3. Fold down the electrical tape.
4. It is difficult to slide the mirror back on to the window mount with the electrical tape covering the springs. I recommend spraying a little silicone lubricant on the tape. If you do not have silicone lubricant, a little oil or even Chap Stick would probably help.
5. Slide the mirror back on to the window mount.
Hope this helps!
Send me an e-mail: intelligentbear @ earthlink.net (you will have to copy & paste & remove the spaces)