It is late, the house is quiet, and there it is: a faint hiss or trickle coming from somewhere inside a wall. Nobody is in the shower, the washing machine finished hours ago, and every tap you can see is off.
Often the answer is harmless. A cistern topping up, the heating pump turning over or a neighbour running a bath through a shared wall can all sound exactly like a leak. But a pressurised pipe escaping inside a wall or under a floor makes the same kind of noise, and it keeps going day and night. This guide shows you how to tell the two apart in about half an hour, using your water meter and your stopcock, and what happens if the sound turns out to be a real leak.
What does a hidden leak actually sound like?
Water escaping from a small hole in a pressurised pipe does not usually drip. It is forced out at mains pressure, so it makes a steady hiss, a soft rushing noise or a faint whistle. The noise travels along the pipe itself and through timber, plaster and brick, so it often seems to come from a wall a few metres away from where the water is actually getting out.
A few features point towards a leak rather than normal plumbing:
- The sound is continuous. It does not stop after a minute or two the way a refilling cistern does.
- It is there at 3am as well as at teatime, when no appliance in the house should be drawing water.
- It is loudest near pipe runs: behind a kitchen sink, alongside a bath, in the wall below a bathroom or where the rising main comes up from the floor.
- It stops when you close the stopcock, which is the single most useful test covered below.
Intermittent sounds are different. A bang when a tap shuts is water hammer, ticking as the heating warms is pipes expanding, and gurgling after a bath drains is air in the waste. None of those is a supply pipe leaking.
How do you check whether it is a leak in about 30 minutes?
You need your water meter, your internal stopcock and a bit of patience. Do this at a quiet time, ideally late evening, when nobody needs water for a while.
- Close everything. Turn off every tap, and make sure the dishwasher, washing machine and any outdoor hose are not running. Wait until every toilet cistern has finished filling and gone quiet.
- Read the meter. It is usually in a pavement chamber or near the stopcock. Take a photo of the dials, including the small red dials or the spinning indicator that shows low flow.
- Wait and read it again. Leave it for 20 to 30 minutes without using any water. If the reading has moved, or the low flow indicator is turning, water is passing the meter.
- Close the internal stopcock and listen. The stopcock is usually under the kitchen sink, in a downstairs cloakroom or under the stairs. Close it fully, then go back to where you heard the noise. If the hissing stops within a few seconds, the sound was water moving through your own supply pipework.
- Check the meter with the stopcock closed. If the meter is still moving with the internal stopcock shut, the water is escaping between the meter and the stopcock, which usually means the underground supply pipe rather than anything inside a wall.
If the meter does not move and the sound carries on regardless of the stopcock, the noise is probably not your mains supply at all. That points to the heating circuit, a stored hot water system or a neighbour's plumbing, which the next section covers. Our guide to checks for unexplained high water usage covers reading meters and bills over several days.
Which harmless things sound like water running in the walls?
| Likely source | What you hear | How to rule it in or out |
|---|---|---|
| Toilet cistern refilling or passing | A short fill that stops, or a faint constant trickle | Lift the cistern lid. Water trickling into the pan or over the overflow means a worn fill valve or flush seal, not a hidden leak. |
| Central heating circulating | A soft rush in pipes while the heating or hot water is on | Turn the boiler off at the controls. If the sound fades as the pump stops, it was circulation. |
| Water softener regenerating | Running water in the early hours for a set period | Check the softener's regeneration time. Many are set to run overnight on a timer. |
| Cold water storage tank filling | Filling noise from the loft after hot water has been used | It should stop once the tank is full. A float valve that never shuts off will send water out of the overflow pipe. |
| A neighbour's plumbing | Running water through a party wall, especially in terraces and converted flats | Your meter stays still and the sound ignores your stopcock. It is worth mentioning to them, because it may be their leak. |
A fill valve that does not quite close lets water trickle constantly into the pan, and the refill noise can carry along the pipework into surrounding walls. If that is what you find, our article on a toilet cistern that keeps running explains the usual fixes.
What if the sound stops when the stopcock is closed?
Then you have fairly strong evidence of water escaping from pipework downstream of the stopcock, somewhere inside the property. Reopen it and check every visible pipe first: under sinks, behind toilets, at appliance valves, around the hot water cylinder and at any outside tap.
If everything visible is dry, the escape is most likely in a pipe that is buried in plaster, boxed in, running through a floor void or laid in a screed floor. In older London houses that is often copper chased into walls or run under timber floors; in newer flats, plastic pipe in a screed.
Look for the slower signs as well as listening. Our guide to the signs of a water leak in walls covers the visual clues: a damp patch or tide mark on plaster, paint that bubbles or flakes, a skirting board that has swollen, a musty smell in one room, or a floor that feels cold or soft in one spot. Hearing it before any of these appear is the best time to find it.
Is it urgent, and what should you do in the meantime?
A constant hiss means water is being lost all the time, and that water is going into the structure of the building. It is not usually an emergency in the way a burst pipe is, but it is not something to leave for weeks either. A slow leak in a flat can reach the ceiling below before it shows in yours.
In the meantime:
- If the sound is loud, or you find any damp, close the stopcock overnight and when you go out, and open it only when you need water.
- Do not start cutting into walls or lifting floors to look for it. The sound can travel a long way along a pipe, and opening up the wrong spot is the most common way a small repair turns into a large one.
- In a flat, warn the neighbours below.
How do we find a leak you can hear but cannot see?
Because the leak is already making a noise, listening for it is the natural starting point. Acoustic leak detection uses a sensitive ground microphone and contact sensors with amplifiers and filters that cut out background noise such as traffic, fridges and boilers. The technician traces the route of the pipe and takes readings along it. The escape sounds loudest directly above the point where the water is getting out, and fades either side.
On longer or buried pipe runs, correlation adds precision. Two sensors are placed on the pipe at either side of the suspected area, usually at fittings or valves. The leak noise reaches the nearer sensor slightly before the farther one, and the correlator uses that time difference, together with the pipe material and diameter, to calculate where along the run the noise is coming from.
Listening is often combined with other methods. Thermal imaging helps on hot water and heating pipes, and tracer gas is used when a pipe is too quiet to hear, such as a tiny escape in plastic pipe that carries sound poorly. More on the kit in how acoustic leak detection works.
The point of all of this is that the repair should mean lifting one tile or one floorboard, or cutting one small square of plaster, rather than opening up a whole wall to follow the sound.
What does it cost to find a leak you can hear?
An acoustic survey costs £150 to £300. Where more than one method is needed, for example listening combined with thermal imaging or tracer gas, a combined survey is £250 to £500. If you want us to find the leak and open up the exact spot so a plumber can repair it, trace and access is £350 to £450. No find, no fee applies to leaks on your own property, so if we cannot locate the escape you do not pay for the survey.
If you can hear water running when everything is off and the stopcock test points to your own pipework, call us on 020 3912 0200 and describe what you heard and what the meter did.
Frequently asked questions
Why can I hear water running in the walls when nothing is on?
The most common causes are a toilet cistern that is refilling or passing water, central heating circulating, a water softener regenerating, a cold water tank refilling, or a neighbour using water through a shared wall. If the sound is continuous and stops when you close your stopcock, it is likely a leak on your own supply pipework.
Can a leak make a hissing sound behind a wall?
Yes. Water forced out of a small hole in a pressurised pipe usually hisses or rushes rather than drips. The sound travels along the pipe and through the structure, so it may seem to come from a spot some distance from the actual leak.
Should I turn off the water if I hear it running in the walls?
Closing the stopcock is the quickest way to test whether the sound is your supply. If it stops the noise, keep the stopcock closed when you are not using water until the leak is found, to limit the damage.
My water meter is moving but the stopcock is closed. What does that mean?
Water is passing the meter but not reaching the house, so it is likely escaping from the underground supply pipe between the meter and your stopcock. That is found with a different approach from a leak inside a wall, usually acoustic listening along the pipe route combined with tracer gas.
Can you find the leak without knocking the wall down?
In most cases, yes. Acoustic listening, correlation, thermal imaging and tracer gas are all non-invasive, and together they narrow the leak down to a small area so that only a small section of plaster, one tile or one floorboard needs to come up for the repair.
