I've posted a similar thread on tmc but I want to share what I think the cause is since I just experience it today. It looks like the belt buckle is hitting the plastic piece behind the seat and causes all these rattles. Try leaning the seat back or move the seat all the way back to see if the rattles go away. I think I just save Tesla some money and you guys time to go to the service center. If anyone finds any kind of padding that can be tape on the seat plastic piece, please share.