Wanted to update my original post to share my experience. Finally isolated the annoying rattle in the B pillar as being caused by the belt lock rapidly engaging/disengaging because of a faulty seat sensor. Took my mechanic for a drive yesterday and he figured it out. He was rocking back and forth with the seat buckled, whether we were moving or not, and the lock relay was fluttering like crazy. So he's going to pull the passenger seat today and replace the sensor. Unfortuntately time consuming procedure so labor cost will be up there but my car will finally be silent. Just wanted to share....!