Why is my drain making a knocking sound?

Plastic moves with temperature, and it moves more than most people expect. Charlotte Pipe, which makes plastic and cast iron drain pipe, describes a "ticking noise" that "occurs when a plastic pipe expands and contracts as a result of temperature changes." Its technical manual adds that "plastic pipe expands or contracts approximately five times more than steel."

Picture a long run of drain pipe resting in a notch in a floor joist or clamped to a stud. Hot water from a shower or a dishwasher goes through, the pipe warms and gets longer, and it has to slide. If a strap is tight, or the hole it passes through is snug, it sticks, then slips, and you hear a tick or a knock. It happens again as the pipe cools and shrinks.

Charlotte Pipe's manual explains the fix in a single line: it recommends "clamps or straps that allow pipe to remain away from the framing, thus reducing the noise generated when pipe is allowed to rub against wood." It also says the pipe "should not be anchored tightly by the support, but secured in a manner to allow for movement caused by thermal expansion and contraction."

One caveat: Charlotte Pipe's article is written about commercial buildings, where long plastic runs make the effect easy to hear. The physics is the same in a house, but a house has shorter runs, so the sound is usually milder.

Time it, and note what you were doing.

• Knock or tick while hot water drains, then quiet. Most likely thermal expansion of a drain (or hot supply) line. It may repeat as the pipe cools off.
• One loud bang at the moment a faucet, valve, dishwasher or washer shuts off. That is water hammer, a pressure surge in the supply lines. Our page on why pipes bang when you shut off water covers arrestors and pressure.
• Gurgling or bubbling from a drain. That sound comes from air moving through traps and drains. Charlotte Pipe notes that "drainage noise is caused by the rush of water and air through traps and drain pipes." Our pages on drain gurgling and a kitchen sink that gurgles when the dishwasher drains cover that.
• Ticking in a wall when you run hot water, far from any drain. That can be the hot supply line. Lubrizol, the maker of FlowGuard Gold CPVC, says its pipe "will expand about 1 inch per 50 feet of straight pipe for every 50°F increase in temperature," so expansion noise can come from supply lines too.

Run one fixture at a time to find the source. Shower alone, then the kitchen sink, then the washer. A noise that follows one fixture narrows the search to that pipe run.

Not by itself. Thermal noise is a sound problem. But a noisy pipe that you cannot see is a reasonable excuse to run a quick leak check. EPA suggests checking your water meter before and after a two-hour period when no water is being used. In its words, "if the meter changes at all, you probably have a leak."

A clog usually shows up as slow drainage, standing water or smells, not as a rhythmic knock. See unclogging a shower drain if the knocking comes with slow draining.

Only what you can reach.

1. 1Find the run. Under a sink, in a garage or in an open crawlspace, look for a strap that is cinched tight, or a pipe pressed against a joist or stud.
2. 2Loosen, do not remove. A strap should hold the pipe, not clamp it. Charlotte Pipe says to "use straps or clamps that allow for piping system movement." Never take away support altogether. The same manual says plastic pipe should be supported "at intervals sufficiently close to maintain correct pipe alignment and to prevent sagging or grade reversal," and to "support trap arms as close as possible to the trap."
3. 3Give it room. If a pipe leans against framing, a strap that holds it slightly off the wood is the manufacturer's recommended approach.

Anything inside a closed wall or ceiling is a different story. Fixing it means opening the wall, repositioning the pipe or enlarging a hole, and that can mean drywall repair. A mild tick in a wall is often something people decide to live with.

Call if:

• the knocking comes with a wet spot, stain, musty smell or water on the floor;
• the noise is loud enough to disturb sleep and you would like it fixed;
• you have a ticking or banging sound that does not fit expansion, or that gets worse over time;
• a drain also gurgles, backs up or smells.

For CPVC supply lines, Lubrizol warns that expansion that is not accounted for "can lead to problems such as cracked fittings and distorted or cracked pipes," and that "these problems can take years to develop." Charlotte Pipe says restraining movement "may result in joint or fitting failure." A plumber can find the run, check the supports and decide whether the sound is harmless or a sign of strain.

HQ Plumbing and Air is licensed and insured, and we give free estimates and upfront pricing. Start with our residential plumbing service if you want someone to trace a noisy line.