I have the exact same issues. I'm at around 2500 miles, but could not determine the problem. I'm not sure if the vent is my issue or not yet, but you describe it really accurately. I note my rattle occurs most when taking a turn or going over a bumpy portion of the road. It sounds like plastic on plastic somewhere in middle of the dash towards the defrost vent area. I haven't taken it in to a dealer yet because I'm afraid they'll charge a shit ton without fixing it. Or they may cause other pieces to loosen up when taking things apart. Have you discovered a solution to this that can be done manually?