Looks like maybe lockbar insert or frame rubbing it. I agree though, take it apart and investigate. I would take some pics of how the blade is sitting in the frame as viewed with knife in same position as you have it there, but swing it around so you are looking into the channel where the blade descends down into the frame. Similar to how folks take pics of blade centering when trying to sell.
Edit - while I was typing I see you already disassembled it.