Final answer:
Water hammer is the rattling of plumbing pipes caused by a sudden reduction in water flow, creating a shock wave.
Step-by-step explanation:
The rattling of plumbing pipes that occurs when water flow is suddenly reduced, causing a shock wave, is called water hammer. Water hammer is a phenomenon that happens when the flow of water in the pipes is abruptly stopped or changed, creating a pressure surge. This surge of pressure travels through the pipes, causing the rattling or banging sound.