Knocking water pipes occur when irregular water pressure in the main supply line enters the home, according to CarneyPHC.com. The pressure is responsible for maintaining water flow throughout a home and into the faucets, but there are times when air leaks from the pipes, disrupting water circulation.
Aztec Plumbing also notes that a more serious form of knocking pipes is called water hammer. This occurs when water makes a sudden stop when the faucet is turned off, spawning a jarring effect that moves the water throughout the pipes in an unstable manner. The instability of the water stops when the energy flow dissipates. According to HowStuffWorks.com, there may be much as 5 to 10 pounds of water that comes to a sudden halt. This also occurs in washing machines and dishwashers, and the hammer effect damages the quality of the pipes.
Aztec Plumbing says the most common causes for knocking pipes are air pockets, disconnected pipes and abnormally high water pressure but notes the problem can be fixed.
CarneyPHC.com recommends shutting off the main valve and flushing out remaining water from faucets and toilets. In the case of water hammer, installing water hammer arrestors lessens the shock of the water within the pipes.