Sounds like this occurs with the gun closed and locked. This would probably be loose solder under the centre rib left over from a poor job of soldering it in place. By itself this won't cause a problem , just sounds bad and gives an indication of the overall quality of the gun. It's also a possibility that you are hearing loose rust scale sliding back and forth under the rib which would be caused by moisture being trapped under the rib due to a poor solder job leaving voids that could allow moisture entry. This could point to hidden corrosion and of course weakness. Re laying a rib is a very skilled gunsmith skill, not within the scope of most Canadian gunsmiths and is an expensive proposition. Have the gun carefully examined by a competent gunsmith to check the rib integrity and possibility of moisture ingress. If it checks out OK you can live with it or move it along.