One of the most annoying rattles in the car - the rear view mirror. Every car I've had suffered from this issue. It's one of the most pronounced rattles because it's directly in the upper center of the sound stage. Super easy to localize - throw on a sub frequency sweep and you'll hear it. It even rattles in the low mid range. You'll know if you are suffering from this issue because when you grab the mirror (like if you were going to adjust it for driving) it will stop. When you let go, it continues rattling.
The cause is the actual mirror is resonating against the plastic mirror housing. Fortunately, this is a no-brainer to fix. Requires some paper and clear silicone caulk.
Step 1: Fold up paper so that it barely fits between the mirror and the housing. My mirror was super tight against the housing so I only folded 3 times but yours may require more folds of the paper to properly space it. Now, push the paper in three spots, two on top, one on the bottom. Or two on the bottom, one on top. Or two on the bottom and two on top. Whatever, the idea here is to push the mirror away from the housing so you can see a gap. You will want to *gently* push the mirror in (towards the engine) so it moves away from the housing while you insert these paper tabs. You have to create a bigger gap than would normally be there or this won't work. On my car, I could not even fit a piece of unfolder paper in there so the fact that I had to fold three times should give you an idea of how big of a gap you need to produce. Again...gently do this, don't break your mirror.
Step 2: Cut the silicone caulk tube applicator just enough to let the product run through it. I like to taper the cut so its easier to apply. You could even just jam a pin through the top so you have a pin hole for it to go through. A nail would pierce too big of a hole. You want this to be as clean as possible.
Step 3: Apply silicone to the perimeter of the mirror. You want the silicone to get into the gap you created with your paper spacers. Skip over the spots with the spacers.
Step 4: Unless you're a surgeon, you probably got caulk where it's not supposed to be. Grab a lint free towel and wipe around the perimeter. The idea here is to clean your mess while also pushing the silicone caulk further into the gaps. Also use the towel to clean any other spots on the mirror where you might have messed up. If you don't clean it now, it will be way harder later.
Step 5: Once you're done cleaning up, immediately remove the spacers. If you did this properly, the caulk will form a small bead where the mirror is now being compressed against the mirror housing. This pushes the silicone out and allows it to bond properly.
Step 6: Wait 24 hours. Don't touch the mirror, adjust it, play music, etc. You need this to dry completely.
Step 7: If you followed all these steps properly, your rear view mirror rattle should be gone. What you've done is coupled the previously free floating mirror against the housing to eliminate the rattle. You shouldn't need to do this again in the future.
Hopefully this helps someone out there. Besides trunk rattles this is easily one of the most annoying rattles in the car and fortunately one of the easiest to fix.
I had already taken the two top paper tabs out before I started taking pics but you can see the gap in the caulk where I skipped over them. This is what you want, don't try to fill those spots in after you pull the tabs or you'll mess up your perfect bead.