If you can hear water running with no taps on, water is moving somewhere it should not be. A steady hiss points to a pressurised supply pipe leaking; a trickle or gurgle usually means a toilet cistern or loft tank refilling; a tick or knock is often pipes expanding. Check the water meter with everything off: if it is moving, the sound is a leak.
It is late, the house is quiet, and there it is again: the sound of water moving somewhere behind the wall, under the floor or up in the loft. Nobody is running a tap. The washing machine finished hours ago. You have stood in the hall with your head on one side trying to work out where it is coming from, and the sound seems to be everywhere and nowhere. Water noise carries through pipes and masonry in a way that fools the ear, so the room where you hear it loudest is not reliably the room where it is escaping. This guide explains what the different sounds usually mean, the two tests that tell you whether it is a leak, and how acoustic leak detection turns a noise into a mark on the floor.
Listen to the type of sound
Water makes different noises depending on what it is doing, and the character of the sound is the first clue.
- A continuous hiss, like a distant kettle or a tyre going down. This is the sound of water under mains pressure forcing through a small hole. It is the classic sound of a leak on a supply pipe: a pinhole in copper, a split in old alkathene, a joint weeping under a floor. It does not stop and does not change with time of day.
- A trickle, gurgle or the sound of a cistern filling. Water falling into water. A toilet cistern topping up because the flush valve is passing, a cold water tank in the loft refilling through the ball valve, or an overflow running. This sound comes and goes, often in cycles, as the valve opens and closes.
- A tick, click or knock. Usually not water moving at all. Copper pipes expanding and contracting as the heating comes on and off, or a loose pipe clip letting a pipe tap against a joist. It follows the boiler, not the taps.
- A bang or thud when a tap or appliance shuts off. Water hammer, caused by a pressure wave in the pipework. Annoying but not a leak, though it can eventually cause one at a weak joint.
- A rushing or roaring in the heating pipes. Air in the system or a pump running dry, often alongside low boiler pressure. That points to the heating rather than the mains, and to a possible heating leak.
The two checks that settle it
The night test
Pick a time when the house will be still. Turn off every tap and every appliance. If you have a loft tank, allow half an hour for it to fill and the ball valve to close. Then walk the house slowly. Put an ear, or the handle end of a screwdriver with your ear on the other end, against the stopcock, the pipes under the sink, the toilet cistern, the airing cupboard pipework and the wall where you hear it most. A hiss that is loudest at the stopcock and fades as you move away suggests the leak is on the incoming supply, possibly outside. A sound loudest at a cistern is the cistern.
The meter test
The meter is the judge that cannot be fooled by acoustics. With everything off, look at the small flow indicator on the meter (in the box near the gate in most Cornish homes). If it is turning, water is passing through with nothing in use, and the noise you hear is that water escaping. Then close the internal stop tap and look again: if the meter stops, the leak is inside; if it carries on, it is on the buried supply pipe. The full method, with the sums for litres per hour, is in the overnight meter test. If the meter is completely still while you can still hear the sound, the noise is not a mains leak. Think heating pipes, air, or something outside the plumbing altogether.
What the sound plus the meter tells you
| What you hear | Meter with everything off | Most likely | Next step |
|---|---|---|---|
| Steady hiss | Moving, stops when stop tap closed | Pressurised pipe leaking inside the house | Locate before opening anything up |
| Steady hiss | Moving, keeps going with stop tap closed | Supply pipe leaking between meter and house | Tell South West Water, locate the leak, check trace and access cover |
| Trickle in cycles | Creeps, then pauses | Toilet or loft tank valve passing | Food colouring in the cistern, check the overflow |
| Tick or knock | Still | Pipes expanding with the heating | Pipe clips, not a leak |
| Rushing in radiators | Still | Air or a heating leak | Check boiler pressure, bleed radiators |
| Anything, but only in wet weather | Still | Rainwater in a downpipe, gully or behind a wall | Check gutters and downpipes, not the plumbing |
That last row catches a lot of people in Cornwall. A downpipe cast into a granite wall, a land drain behind a cob cottage or a stream culverted under a village street can all sound exactly like a leak indoors after rain. The meter test rules the plumbing out in a minute.
Why the sound is so hard to place
Sound travels along a copper pipe far faster and further than it travels through air. A leak under the kitchen floor can be heard most clearly at a radiator upstairs because the pipe carries it there. Masonry does the same. A hiss in a supply pipe under a slate floor will resonate through the whole slab, so the noise fills the room with no obvious source. In a thick-walled Cornish cottage the effect is stronger still, because the walls carry sound well and the floors are often laid straight onto the ground with the pipe underneath.
This is why cutting into the wall where the sound seems loudest so often finds nothing. The sound is real; the location is an illusion.
How acoustic listening turns a noise into a spot
Acoustic leak detection works on the same principle as your ear against the stopcock, with equipment that can hear what you cannot and, more importantly, compare how loud the leak is at each point along the pipe.
- A ground microphone is placed on the floor, path or lawn at intervals along the pipe route. The leak noise is loudest directly above the escape point and falls away in both directions, so the peak marks the spot.
- A contact probe is touched to accessible pipework, stop taps and meter fittings to pick up the leak vibration carried in the metal and follow it back.
- Filtering strips out traffic, wind and the boiler so the leak frequency can be isolated. Quiet is still helpful, which is why surveys on busy roads or in windy coastal spots are sometimes done early in the day.
- Correlation, on longer runs such as a farm supply pipe, places two sensors on the pipe and works out where the leak is from the delay between them hearing it. It is how a leak on a long run to a farmhouse near Bodmin is found without walking every metre.
Where the ground is noisy or the pipe is plastic and carries sound poorly, tracer gas is used instead: the pipe is charged with a safe gas mixture that escapes at the leak and is detected at the surface. Thermal imaging adds a third view where a cold supply pipe or a warm heating pipe has wet the surrounding material. Together they give a location to within a metre, and a written report describing what was found and where.
When the sound is the heating, not the mains
If the meter is still and the sound follows the boiler, the water you hear may be in the heating circuit. A sealed system does not draw from the mains once filled, so a heating leak never moves the meter. Its symptoms are pressure that keeps dropping on the boiler gauge, radiators that need bleeding again and again, and sometimes a rushing noise as air circulates. The guide to central heating losing water with no visible leak covers where that water is going. The listening method is the same; the pipe being listened to is different.
What to do tonight
- Note when you hear it, what it sounds like, and where it seems loudest.
- With everything off, check the meter’s flow indicator. Photograph it.
- Close the stop tap and check the meter again.
- Food colouring in each toilet cistern; look for colour in the pan after twenty minutes.
- Check the boiler pressure gauge if you have a sealed system.
- Walk outside and look at overflow pipes and the ground along the supply pipe route.
If the meter moves and the toilets are clean, the sound is a hidden leak, and the sooner it is located the smaller the damage. We provide leak detection across Cornwall, and a night-time hiss is one of the clearest starting points a survey can have.
Frequently asked questions
Is hearing water running always a leak?
No. A toilet cistern refilling, a loft tank topping up, pipes ticking as they warm and rainwater in a downpipe can all sound like running water. It becomes likely to be a leak when the sound is continuous, does not follow any tank or toilet, and the water meter moves with everything switched off.
Why can I hear it in one room when there is no pipe there?
Sound travels along copper pipe and through masonry much further than through air, so a leak is often heard loudest well away from where it is. The room where it sounds loudest is a poor guide to the location, which is why listening equipment compares the sound at many points rather than trusting one.
Can a leak make a sound without the meter moving?
Yes, if the leak is on the central heating. A sealed heating system does not take water from the mains while running, so a heating leak never turns the meter. The clue there is falling boiler pressure. A waste pipe leak also will not move the meter, but only makes noise when water is being used.
Does acoustic leak detection damage anything?
No. The microphones and probes sit on the floor, the ground or the pipe surface. Nothing is opened up to listen. Any opening happens afterwards, at the point the survey identified, and is as small as the repair allows. That is what trace and access cover is designed to pay for.
How quiet does the house need to be for a survey?
Quieter is better but silence is not required. The equipment filters out much of the background noise, and the operator chooses times and positions to work around traffic, wind and the boiler. Turning off appliances and the heating during the survey helps, and we will ask for that on the day.
Hear it but cannot find it?
We listen along the pipe with equipment that hears where the sound peaks, and mark the spot before anyone lifts a floor. The report at the end supports a trace and access claim on your buildings policy.
