Update. Service center replaced both rear seat backs and the rattle/creak is completely gone. Feels like a brand new car, some other rattles disappeared as well. I’m so happy.Ok so it’s either the latch in the seat or where it mates. What you can do if you don’t have anyone in the back of your car often (and this is just a for-your/sanity-fix) is to wedge the cloth in the side so that you can put the seat up and wedge it “closed” but not latched. At least you’ll be able to drive without losing your mind. I’m sensitive to anomalous sound too, so… I feel your pain.