Noise by drivers side seat belt adjuster

Hmm, I doubt it's the airbag. By "seatbelt adjuster" do you mean the anchor point on the B pillar above your shoulder that you can move up and down? (I assume it's like that in the Prime ... correct me if I'm wrong, as I'm extrapolating from a Gen 3.)

If you squeeze your ears in between the seat and the pillar, can you pin down that the sound seems to come from the direction of the seat or the direction of the pillar?

 

Does the noise happen at any time, only when the car is moving, or only when the car is stopped?

Does the noise happen only with the car in READY, or also in the other power switch modes (Ignition On, Accessory, or Off) (Owner’s Manual (PDF), page 334)? If you test this, I’d suggest waiting a minute or so after each change of mode, in case whatever’s causing the noise takes time to start or stop.

Does the noise change if you turn the air conditioning fan off?

 

I used to have a car that did that and it took me awhile to figure out it was the seat belt pulley. Every time my body shifted just a little the pulley would squeak. When it's very quiet in the car try pulling and releasing the seat belt just an inch or two ... slowly.

 

The pillar trim is usually not at all hard to remove. I don't have a Prime, but on my Gen 3 it's just a matter of pulling the rubber door opening weatherstrip away from the pillar edges and the bottom (sill) edges, popping up the plastic sills for the front and rear doors, then popping out the lower pillar trim. On mine that has to be first because it exposes a little bolt at the bottom of the upper pillar trim, which can then also be popped out once that bolt's removed.

That should give you easy access with a mechanics' stethoscope (they're cheap) to everything back there.

You may notice some things with wiring connected to them that is bright yellow. Don't disturb any bright yellow wiring while removing trim or poking around for the noise. Yellow wiring is part of the airbag and seatbelt-tensioning systems, and any safe manipulation of that wiring requires removing the car's 12 volt power and waiting a prescribed period. You shouldn't need to mess with any of that just to pinpoint where the noise is coming from.