I actually had this same thing happen to me (22 Refresh TB) when my truck had about 400 miles on it. For me, it was the driver side rear back rest that was making the noise. If I pushed on the area where somebody’s left shoulder would be (if they were sitting in the seat), I could get it to make the noise.
The dealer ended up taking the seat back apart (had to remove the head rest and a couple of latches to get the seat to fold down, I believe), and when they put it back together, it didn’t do it anymore! I think there was some insulation that didn’t get set right behind the seat from the factory that they put back the right way when they reassembled it. Ever since they did that, it’s been great.