Banging occurs in steam radiators that are trapping water inside. The extra water is picked up by repidly moving steam and slammed against the other end of the radiator. Pooling of water inside the radiator can be caused by improper pitch (slant) away from the pipe that allows the water to flow down back to the boiler. Rust and dirt can collect in the bottom to create a barrier to water flowing out of the radiator. Two-pipe steam systems had steam traps at the bottom pipe of the radiator that can malfunction to resist water flow out of the radiator.