Federal regulations require that all propane tanks are fitted with an OPD (Overfill Protection Device). The OPD feature prevents accidental overfill of the cylinder. An internal float mechanism shuts off the valve when the propane tank is 80% filled. The 20% empty space is necessary to prevent the cylinder from venting large amounts of propane when the temperature rises. The OPD valve also contains another safety feature - it will not release gas unless the pigtail hose is properly connected, even with the valve open. These devices are metal, and some tanks have a more pronounced "clang" when the tank is moved around (depending on the manufacturer of the tank). Nothing is broken and the sound you hear does not have an impact on the function of your tank.