Who's That Knocking?

Knocking sounds from the wall can become really irritating. It's a problem that many homeowners experience, particularly in older homes. This reader asks:

"I have a strange knocking sound coming from the wall in my bathroom. I think it may be the pipes but I don't know what I can do about it."

Air chambers cushion the shock wave from shutting off water.

Fast moving water coming to a sudden stop will cause a shock wave. This shock wave will cause the pipes to move and vibrate causing a knocking sound when the water is turned off. This effect is commonly called a "water hammer". Water pipes usually have a short, specially designed piece of pipe called an air chamber to absorb the shock and prevent this from happening. What could be happening is that there either is no air chamber or it has filled with water. If there is an air chamber in place, it should be located near the faucet. Shut off the water to the faucet, drain out the air chamber and put it back in place. If there is no air chamber then installing one should stop the pipes from making the knocking sound anymore. Draining an air chamber is a pretty straightforward process for anyone comfortable with plumbing but some homeowners may feel more comfortable calling a professional. Installation of an air chamber is a process that is usually best left in the hands of a plumber.

In some cases, knocking can be coming from metal heating and cooling ducts. This happens because the ducts are expanding or cooling as the furnace switches on and off. There isn't a whole lot that can be done about this problem although a heating and cooling (HVAC) company may be able to offer some additional insight and assistance.