I know that on my prior 2009 c300 sedan the rear deck shelf had a rattle. It was the mesh being a bit loose and rattling. My brother's coupe has this same issue, mine does not. Back then on my sedan, the remedy was a bit of felt, you can also try to access it from underneath via the trunk. Some people have had success getting felt in that way, or foam padding to help stop the rattling.
Another source of rattling which I have located in my coupe (which you may be confusing with your rear deck shelf) is that the passenger seat head rest sometimes rattles slightly, even on "ideal" road surfaces. Although, I have really only noticed this when no one is in the seat.