I will get a rattle from the B pillar if the seat belt buckle is not retracted all the way.
It will bounce between the top of the seat back and the pillar.

Hope this helps.

 

My passenger seat used to rattle like crazy. Now is has calmed down. Sometimes the passenger seat belt female side rings when nobody is sitting there so I push it down between the seat and the center console and it hushes it.

 

Hello Everyone, thanks very much for you support on this. unfortunately I was not able to find the source, and sold the car today.

Thanks again..

 

Wow. That must have been one intense rattle.

Good luck.