Yeah... it was great relief because tearing into the seatbelt assembly looked to be a birch and the sound ( incorrectly) led me to believe that's what it was. The "fix" you're going to do May/may not provide relief. Don't give up. Try confining it at different angles and tensions until resolved. Mine is strapped down very tightly with two ties holding it on two different planes.
It's hard to imagine that the rollover valve would rattle so loudly, but I will definitely give it a shot. Is it the valve itself or is it the hose connecting to the valve that needs to be secured? I can't remember what it looks like, and in the photo that Richard so kindly posted, it looks like the arrow is pointing towards the hose.
