I hear dripping or running water but cannot find a leak. Where is it coming from?

It is a small sound. A drip somewhere behind a wall, or a faint rush of water when nothing is running, and once you notice it you cannot stop hearing it. Sometimes it is a leak. Often it is something else that sounds like one. The way to find out is to stop guessing and work a list in the same order every time.

First, is water actually leaving the pipes?

The meter answers this in fifteen minutes and saves an hour of crawling under sinks.

1. Turn off every faucet, the irrigation, and the icemaker if you can reach the valve, and make sure no toilet is filling.
2. Go to the meter at the street, lift the lid, and look for the low-flow indicator: a small triangle, star, or wheel on a dial meter, or a flow icon on a digital one.
3. Watch it for a minute. Then write down the reading and come back in fifteen minutes without using water.
4. If the indicator is turning or the reading changed, water is leaving a supply line somewhere, and the sound is real.
5. If the meter holds still, no supply line is leaking. The sound is coming from a drain, the AC, an appliance, or a fixture that only runs sometimes.

Close the main valve at the house and check once more. If the meter stops, the leak is inside; if it keeps moving, it is in the yard line between the meter and the house.

The usual suspects, in order

• Toilets. A worn flapper lets water seep into the bowl and the fill valve tops the tank back up every few minutes. That refill is the classic “running water” sound in a quiet house. Put food coloring in the tank and wait ten minutes without flushing; color in the bowl is your answer.
• The water heater relief line. The temperature and pressure relief valve on the side of the tank discharges through a pipe to the floor or outside. A valve that has started to weep drips at the end of that pipe, sometimes only after a heating cycle. Look at the end of the line and the floor under it.
• The AC condensate line. In our humidity the air handler makes gallons of water a day, and it drains through a PVC line, usually out a side wall or into a drain. Water dripping from that line outside is normal. Water in the drain pan under the air handler, or a drip from the ceiling below an attic unit, means the line is clogged with algae. It sounds exactly like a plumbing leak, it is not one, and it is one of the most common false alarms we see.
• Irrigation. A zone valve that does not close all the way runs water to the yard around the clock, and the sound carries through the pipe into the house. Check the sprinkler heads for a wet spot.
• The refrigerator line. The icemaker line behind the fridge is a small tube with a small valve, and it leaks quietly. Pull the fridge out and look at the floor.
• Under the sinks. Open every cabinet. Look at the supply line connections, the shutoff valves, and the trap. A drip from a supply line will show on the cabinet floor; a drip from the drain only shows when the sink is used.
• The washing machine. The hoses, the valves behind the machine, and the standpipe. A hose that drips only under pressure leaves a stain under the machine.
• The attic. On a slab home that has been repiped, the supply lines run through the attic, and so do the AC drain and sometimes a water heater. A leak up there stains the ceiling below before it does anything else. Look up in every room.

Give each one a minute. Most of the time the sound has a name by the end of this list.

When the meter moves but nothing is wet

This is the case that needs a different approach. Water is leaving the system and going somewhere you cannot see: into the sand under the slab, into the block of an exterior wall, or into attic insulation. A few things you can do:

• Feel the floors. Walk the tile barefoot. A warm patch is a hot-side leak under the slab.
• Look at the bottom of every wall. Swollen baseboard, a stained paint line, grout that stays dark.
• Check the outside. A wet spot in the yard or against the foundation on a dry day, or a section of stucco that is darker than the rest.
• Isolate hot from cold. Close the cold inlet valve on top of the water heater. If the meter stops moving, the leak is on the hot side.

A moisture meter, the pinless kind sold at hardware stores, will show elevated readings in drywall or wood near a hidden leak, and it can narrow a whole wall down to a few feet. What it cannot do is find a leak under three inches of concrete.

Where DIY stops

You can do the meter test, the toilet test, the walk-through, and the moisture meter. What you should not do is open drywall or cut a floor on a hunch. A wall opened in the wrong place is a repair on top of the leak, and slab work is not a homeowner project at all.

When the meter moves and the list above has not found it, the next step is electronic leak detection: isolating the lines, pressure testing each one, listening with an acoustic device for the hiss, and using a thermal camera to see the warm plume on a hot-side leak. That gives a mark on the floor or the wall within a foot or two, and the wall or the slab is opened once, in the right place. It is a specialized visit, and it is the part of this job that turns a mystery into a repair.

If you have worked the list and still have a sound and no source, send us what you found: the meter result, whether the floor is warm anywhere, and where the sound is loudest. That is usually enough for us to tell you what kind of leak it is likely to be and what the next step looks like.