I have the same issue. I thought it was the buckle rubbing up against the center console. But when I removed the center console to put in some wiring it continued to squeak. It sounded like it was coming from inside the seat in the buckle location.
Had a similar problem on my Tundra. Couldn't nail down the source. After a couple trips to the dealership, they found it. Seat rails need to be tightened to the deck. Simple.