I have the same issue. I just bought your same car around the time you did. After disassembling the pillar and holding my ear up to it while driving (I do not recommend doing this), I think I traced it to a small piece of styrofoam on top of a thin piece of plastic in the pillar right below the silver seatbelt holder loop. I tried squirting water on the foam, which actually worked. The only issue is, the next day (after I assumed the water dried up), the rattle was back. I'm going to try more things and I will update here if I figure it out.