I wanted to share with you what I did to shut my seat rattle up. Just in case it will help you. I don’t have pics yet but I was able to isolate the plastic piece that was rattling when my passenger seat shakes. If you can locate the long plastic trim piece that runs along the outside seat track/rail between the end caps, that was the piece making the noise/rattle for me. It’s the piece that almost looks to rest along the carpet but actually sits in and around the seat rail. Nearest the door. I could mostly reproduce it by banging on my seat back to simulate bumps in the road. I just put my ear down there and banged on the seat back while parked. Seats shakes pretty good. When I held that piece firm the seat still shook but the plastic rattle was minimized a lot. Almost silent. I couldn’t totally remove that trim piece but if you pull the end caps you will see how you can loosen it up by releasing a clip on the front side. I loosened it up and applied felt along the top of the trim piece and resecured it. Rattle seems to be gone. I wish the seat was more secure but at least the rattling is solved. You may have a different cause for your rattle but you might consider this if you haven’t yet. Hope it helps.