I believe that someone a few years ago having had a similar problem, found that the mic behind the headliner was loose. There is a way to pull down the cover to check the mic for looseness. It is supposed to snap into place. There is a Youtube video showing how to drop the cover and how to snap the mic in place. I'm sorry that I don't remember the title of the video. Maybe someone else can supply it.
EDIT: I found a video that shows how to drop the panel and how the mic is clipped to the panel. Start at about 1 minute to see how to drop the panel. https://www.youtube.com/watch?v=goLjp3krjfU
EDIT: I found a video that shows how to drop the panel and how the mic is clipped to the panel. Start at about 1 minute to see how to drop the panel. https://www.youtube.com/watch?v=goLjp3krjfU