I finally found the culprit that was causing the rattle in the mirror area of my 2015 RT which was driving me crazy. On the base RT there are two hole covers on the instrument cluster facia that when removed will accommodate analog gauges that are standard on the more expensive RT models. I removed the facia and found plugs that are used to power the analog gauges (when installed). The plug on the right side was rattling around generating a very annoying noise. I cut some strips from an old tee shirt and wrapped both plugs. Problem solved.