That anoying rattle drove me crazy, and i was reluctant to take out the headliner. It's the plastic tube that runs from the left rear corner of the sunroof down the left edge of the roof and into the trunk, it ends up right where the jack is located behind the panel in the trunk on the left side. now this tube is about 1/2 inch (13 mm) in diameter.
So here is the fix. Get a peice of wire like a coat hanger size, solid like a coat hanger. And file the end of it smooth and slide it up the tube. This will keep the plastic tube from floppin around. Worked for me!!!