Annoying Plasticky noise from rear seat area

[NYHeel]

So I have a very annoying noise in my M3 RWD where it sounds like I have a stack of plastic water bottles banging against each other. I’ve had the noise for about 2 months now and it’s not getting better. If anything it’s getting worse. The interesting thing is that I had a loaner model Y a number of months ago back when my 3 was getting painted and the loaner made the same/similar noise.

Here’s a video with the noise. I actually got the noise to occur in the second half of the video by accelerating sharply and then breaking sharply. I didn’t even need to hit a bump.

 

[XPsionic]

In my car I find that the seat backs are slightly loose in their upright and locked positions, and all the vinyl rubs on each other creating small plasticky noises. Putting foam on the back of the seat backs so they lock into position more securely helps for a few days, until the foam compresses and the noise is back. Maybe cloth sheets in between all the areas were the vinyl could help? Or maybe something more solid rather than foam that forces the seat backs to stop moving/vibrating would help?