If you fold the rear seat down, and the "Creak" goes away, it's a known issue that often pops up after a month or two. Tesla can make a temporary fix, but it may be quicker to just do it yourself. Some owners wrap electrical tape around the "U" metal latch post as a fix (I think Tesla does the same). I stuffed some stiff foam in the "U" post to keep the seat from rattling. So far (a few weeks) it's been rattle free. No word if Tesla has a more permanent fix.