I had this problem in my Model S - noise from the bottom anchor of the driver-side belt rattling against the lower side plastic of the seat while driving. It's very dependent on the seat position, so no noise from passenger side. I too stuck a self-adhesive felt pad on the inside of the lower anchor point - a 2" diameter pad does the trick.
Since there's a big bump (rivet) on the backside of the belt anchor, I first cut/scored an "X" into the center of the pad so it would bend and adhere nicely over the center of the rivet. wish I had a black felt pad, but oh well it's out of sight and no more noise.
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