I'm so lost with finding rattles in my car, ill try almost anything now. How'd you remove the piece?
I also found a rattle in this piece by the feet on the front door, you get to it by removing a smaller piece above it by the dashboard, and I forgot how I did it after that, i think i removed the door rubber piece, I took out the wire harnesses by the baseboard too and added double sided tape over anywhere 2 pieces of plastic touch, and just by tapping that one panel I'm pointing at, it rattles by itself, shoving double sided tape where the piece rattles fixed it, but I shoved too much in and broke it, luckily it's on the inside and not noticeable. Still need to do the passenger side.
My dashboard rattles on the passenger side and driver side too now... maybe that's a call in to tesla fix, because I'm not sure I want to open that.