same thing here & now i've found what it is
i've been trying to locate the same irritating noise form the pillar as well and under the same driving conditions. mine is not really a rattling sound but rather more of a knocking "clunking" sound. i've been fiddling around with the seatbelt upper post thinking that was it. After checking all the nuts and putting extra plastic strips to tighten the moving parts it was still there. that really irritated me more.
...then just this afternoon i found what it was. i suspected that before but i just dismmissed it.
it was the locking mechanism on the door. just to prove it, i wrapped the silver metal (w/ the black plastic bolted on the pillar where the door lock engages) with an electrical tape to tighten the hold. well true enough, the locking must have been a bit loose as the sound never came out again.
i guess i have to adjust the door lock mechanism so that will have a firmer hold when the doors are closed.
try it on your car and hopefully you'ld solve that mistery sound.