The clicking noise that proceeds from hot-water pipes after hot water has been drawn is caused by the expansion of a pipe (or pipes) when suddenly heated. If the pipe is cold, as is probably the case, before water is drawn off, it becomes suddenly hot when a tap is opened, and begins to expand lengthways. Wherever the pipe is so tightly fixed that free expansion is impeded, the resistance is overcome with a little jerk that causes the clicking noise. Pipes laid under floors across joists, where notches are never cut very deep, often give out the noise described. The same thing sometimes happens with circulating pipes as well as branches, though, in this case, it may be the sudden cooling and contraction of the pipes that cause the noise.