Sorry for multiple replies on two different topics but when you say "shake it" you're referring to the part pictured in the OP? FWIW, I feel like I can duplicate the sound by pressing on the chrome piece that's inside the part pictured. There's a chrome loop which the seatbelt goes through in that opening and it kind of flexes back and forth and side to side to allow the seatbelt range of motion. If I press it, I can make it click a bit which sounds like the same noise I hear when I'm driving. If when I'm driving, I press that chrome piece down and hold it, the clicking seems to stopSometimes the lock mechanism rattles around. The only thing above there is the same seatbelt point the OP mentioned. Go to your rig and open the door and shake it. If no noise pull in the seat belt anchor. Anything above that is in the headliner.