Interior buzz/rattle fix: seat belts
 

I think I finally fixed/reduced the noise that's been in my ear for 6 years: seatbelt mount noise.

I've tried multiple times to silence it.

I think I finally figured it out: Seatbelt mounting bolt to D-ring vibration.
Both of these parts are metal (no surprise, need strength for impact).
The seatbelt bolt has a metal doughnut spacer to allow the D-ring to rotate even when the bolt is correctly tightened. But this leaves a tiny bit of play and the two metal parts can vibrate together.

My current solution (other suggestions welcome!)
1. Remove cover & seatbelt mounting bolt (Torx + big wrench)
2. Remove lock washer that holds bolt inside seatbelt D-ring ( I got it started with tiny screwdriver and needle-nose, then could just thread it off the bolt)
3. bolt & spacer slides out of D-ring
4. Cut a "washer" of reasonable size from closed cell foam sheet (leftover from sound deadening project, e.g. https://www.sounddeadenershowdown.co...-cell-foam-ccf)
5. Washer can be oversized, since trim piece will cover it all.
6. reassemble with new washer between bolt head and D-ring

I'm hoping the closed cell foam will last a while, it's not designed for motion/friction so it may tear appart over time as the seatbelt moves.
But the thickness of it holds the D-ring and bolt quiet.

So far, I can't tell any rattle next to my ear.

Unfortunately it's taken me so long to fix that the rest of the car, particularly passenger side dash is rattling loudly. Arg.