Why do I hear running water in my walls when nothing is on?

Start with the causes that need no repair at all, then work toward the ones that do.

Normal sounds. Plumbing moves water for reasons you don't see. A water heater refills after a shower or a load of laundry. Pipes shift as they warm and cool. Rheem's guide to water heater noises lists sounds that are part of normal operation, including "Ticking or clicking: Often caused by heat traps or pipe expansion as temperatures change" and "Gurgling: Normal water movement after periods of heavy use." Rheem adds that quiet noises at regular intervals are usually fine: "If these sounds are quiet, brief and at regular intervals, they're generally nothing to worry about." For the ticking side of this, see why pipes tick when hot water runs.

A toilet that leaks into the bowl. This is the most common cause of a faint trickle you can hear from the next room. A worn flapper lets tank water slip into the bowl, and the fill valve tops the tank up again, over and over. The EPA notes that "Common types of leaks found in the home are worn toilet flappers, dripping faucets, and other leaking valves." Chandler's leak guide gives a quick check: add a few drops of food coloring to the tank and wait. "If the water in the bowl changes colors, the rubber flapper needs to be replaced." If that's your sound, how to fix a running toilet walks through it.

A leak in a pressurized supply line. A pinhole or a loose fitting inside a wall can hiss or trickle around the clock, and it will keep going after every fixture is off. This is the one that does real damage, because the water has nowhere to go but into drywall, insulation and framing.

Something that fills or runs on its own. Movement on a water meter can come from more than a pipe leak. Chandler's guide says to "Consider that movement in your meter can also be caused by things like an automatic pool filler, a leaky irrigation valve, or an evaporative cooler." An ice maker is another appliance that draws water without anyone at the tap.

High water pressure. A whistle or rush that comes and goes can be pressure that's too high for the plumbing. The City of Phoenix says "Water pressure greater than 80 psi requires action by the home or business owner as it can damage your water pipes and appliances." If you suspect this, read about signs your pressure regulator is going bad.

You don't need tools. You need quiet and your water meter.

1. 1Turn off everything that uses water. Chandler's instructions spell out what that means: "Turn all water-using appliances off so that no water is being used. This means turning off all water inside and outside the house including showers, sinks, washing machines, ice makers and any appliance that uses water. If you have an automatic irrigation system, turn off the controller."
2. 2Read the meter. On a digital meter, Chandler says to "Observe the flow rate screen for at least 10 flashes. If the number is greater than zero on any of the flashes, you have a continuous leak." On a dial meter, watch the sweep hand and any small flow indicator, note where it sits, wait 10 minutes and look again.

If the meter doesn't move and the sound is gone, it was ordinary plumbing behaviour. If it moves, water is going somewhere. Our full walkthrough is in how to do a water meter leak test, and the wider set of checks is in how to detect leaks in your home.

Narrowing it down. Now shut the main valve to the house. Chandler explains the logic: "If the leak indicator or dial is still moving, water is flowing between the meter and the shut-off valve." With the main closed and the meter still turning, the leak is outside, on the service line. If the meter stops with the main closed, the leak is inside the house, downstream of the valve. Leaks past the meter on private property are your responsibility, since Chandler states "Any leaks occurring after the meter on private property are the responsibility of the homeowner."

Then listen. Put your ear near the wall, or a glass against it, and check whether the sound is loudest near a toilet, the water heater, a laundry box or a wall where a pipe runs. A sound that follows a single spot points to a fitting. A sound near the water heater deserves the next section.

Rheem's guide is direct about the difference between normal noise and trouble. It says "a hissing, sizzling or dripping water heater can be dangerous," and lists possible causes such as water dripping onto a hot burner on gas models or a leaking heating element gasket on electric models. Its advice for those sounds is to turn off power or gas to the unit and avoid using it until it's inspected. A rumble or pop is different. That is usually sediment, and our page on water heater popping and rumbling covers it. Banging is usually water hammer, covered in why pipes bang when you shut off the water.

Stay calm and move in order.

1. 1Shut off the main water valve.
2. 2Open a faucet at the lowest point in the house to relieve pressure, if you can do it safely.
3. 3Look for the tells: a stain, a bubble in paint, a warm or cool patch, a musty smell. Our page on hidden signs of a water leak has the full list, and water leaking from the ceiling covers the wet-ceiling case.
4. 4Don't cut into a wall yourself unless you know what else runs through it. Electrical, gas and low-voltage lines share those cavities.

Speed matters here because of mold. The EPA says "If wet or damp materials or areas are dried 24-48 hours after a leak or spill happens, in most cases mold will not grow." That's a short window, and a leak you can't see is one you can't dry.

Call one when the meter moves with everything off, when the sound is a hiss you can't trace, when a wall or ceiling is damp, or when you hear water and can't find any of the causes above. Finding a hidden leak doesn't always mean opening a wall, and how plumbers find hidden leaks explains the methods. HQ Plumbing & Air is licensed and insured, gives free estimates, and quotes the price up front before work starts. A leak inside a wall is an emergency, and we're available 24/7.

If the meter holds still and the noise fades, you've learned something useful too. It was the house being a house.