I spent countless hours chasing this, and blew the entire b-pillar apart multiple times. I've lined everything with felt to cushion/remove play, and ultimately found the noise was coming from the rubber moldings around the doors.
Pull the moldings off on the car side (you would remove them to remove the interior panels around the seatbelt adjuster anyway), and spray silicone lube along the inside of the channel for the entire strip. This should solve your problem, as it did for me.
I also have some creaking from the handle overhead, and I've used felt to remove the play in it. I just noticed it's making noise again over hard bumps this morning, so I'll probably use silicone lube to try and attack it. I'll report back with what I find.