For a while I thought there was something loose inside the driver's side B-pillar and it was extremely annoying. Every time I went over bumps or simply gave the gas pedal a little more pressure, I'd hear a clacking noise coming from the B-pillar area. The Premium ESP unfortunately doesn't cover rattles, so I was getting ready to use a third party mechanic, until a friend helped me diagnose exactly where the noise was coming from.
It wasn't actually the B-pillar. It was the bottom part of the seat belt, the piece that attaches to the floor (circled in red in the attached photo). It was constantly knocking against the plastic bottom part of my seat, producing the rattle that was driving me crazy for over a month.
I've solved the problem for now by sticking a piece of soft foam to the plastic bottom part of the driver's seat, so that the seat belt knocks against that instead and produces almost no sound. It works, but it's kinda ugly and eventually that double sided tape will lose its stickyness and the foam will fall off.
Anyone have a more elegant and permanent solution?