I've had rattles.
First: Check your sunglass holder. I put in a pair of sunnies, forgot about them, had mysterious rattles. Had my daughter point out the rattle from the passenger seat, removed the sunnies - rattle gone!
Second: When passenger seat was at lowest position and no one sitting in it, the wires underneath rattled. Passenger seat slightly up and rattle gone.
Third: When driving on corrugated dirt roads I got a seriously loud rattle from the instrument panel (well we all agreed that's where it came from). Back in the city, no rattle. As the car was full of camping gear it is difficult to say for sure that it didn't come from any gear - still wondering, maybe the radio wires as noahchanin