Why do radiators clank?

Answer 1

Two words: thermal expansion. When heating units warm up, the material they are made of expands. In the "old style" radiator systems where steam is plumbed up through the heat exchanger (the radiator), all that steel piping and the joints, valves, traps and such get "bigger" by just the tiniest amount. And because the system is rigidly mounted (with some "bumpers" or the like in many cases), it makes noise when it "increases in size" by even the smallest amount. A bit of thermal expansion puts a small amount of pressure on a joint or a run of pipe, and then it "jumps" a bit when the mounts let loose. Solids in general and metals in particular are exceptionally good at conducting the mechanical energy that is sound. You can hear conventional Plumbing in a lot of structures when anyone in the house turns on a tap. Take this across to consider the sounds in steam heating systems and you've got a handle on it.