Toilet Making Noise After Flushing? What Each Sound Means

Most noises after flushing come from the cistern refilling. A hum, whistle or foghorn sound while it fills usually means the fill valve, the float-operated valve that lets water into the cistern, has a worn or scaled washer or diaphragm, or is struggling with high mains pressure. A bang or knock when the filling stops is water hammer. A hiss that carries on after the cistern should be full means water is still running, either through a fill valve that won't shut off or past the flush valve into the pan. Gurgling in the pan after the flush is a drain or vent problem, not the cistern. Listening to when the noise starts and stops tells you which one you have.

What each noise means

Noise	When	Most likely cause
Hum, drone, whistle or foghorn	While the cistern refills	Worn, hardened or scaled fill valve washer or diaphragm, or pressure too high for the valve
Bang, thud or knock	Just as the cistern stops filling	Water hammer from the valve closing, made worse by loose pipes or high pressure
Hiss or trickle that keeps going	After the cistern should be full	Fill valve not shutting off fully, or a worn flush valve seal letting water into the pan
Short hiss every so often, then a refill	With nobody using the toilet	Water leaking slowly into the pan, so the cistern keeps topping itself up
Gurgling or bubbling in the pan	After the flush, or when other appliances drain	Partial blockage or a blocked vent in the soil pipe
Very slow refill with a thin whistle	While filling	Part-closed isolating valve, or a blocked valve filter

Humming, whistling or a foghorn noise

This is the most common toilet noise. As water passes the fill valve, a worn, hardened or scaled rubber washer or diaphragm can vibrate, and the cistern acts like a sound box. It often gets louder over time, and it can be loud enough to hear in other rooms or the flat below.

On older side-entry ball valves, noise can also come from a valve fitted with the wrong seat for the water pressure. Valves for high-pressure mains supplies and for low-pressure tank supplies use different-sized seats, and the wrong one can make the valve noisy or slow.

What helps:

• Turn off the isolating valve on the cistern supply, flush to empty the cistern, then take the top off the fill valve and clean out any grit or scale. Some valves have a replaceable diaphragm or washer.
• If cleaning doesn't cure it, replace the fill valve. Modern valves are quieter and cheap, and on most close-coupled toilets it's a straightforward swap. Concealed cisterns behind a wall panel take longer to get to.

The Water Fittings Regulations require a servicing valve on the inlet pipe next to every WC cistern, which is what makes this job possible without turning off the whole house. Our guide to the toilet isolation valve shows where it is and how to use it.

Banging or knocking

A single bang or thud just as the cistern finishes filling is water hammer. When the fill valve shuts, the moving water in the pipe stops suddenly and sends a pressure shock back along it. If the pipe is loose, or rests against a joist or boxing, it knocks. It's often worse at night when mains pressure is higher, and in houses with long, poorly clipped pipe runs.

Fixes include clipping loose pipework, replacing the fill valve with a type that closes more gradually, and dealing with high incoming pressure if the rest of the house also bangs when taps are shut. Our guide to water hammer explains the causes and fixes in more detail. Don't ignore repeated banging, because the shock loads the joints each time.

Hissing that won't stop

If the cistern should be full but you can still hear a hiss or a trickle, water is still running. There are two usual reasons:

• The fill valve isn't shutting off fully. The float may be set too high, so water keeps running into the overflow, or the valve's seal is worn. If water is running out of an overflow pipe outside, this is the likely cause.
• The flush valve is letting by. A worn seal at the bottom of the cistern lets water trickle into the pan. The cistern level drops, the fill valve opens briefly to top it up, and you hear a short hiss every so often with nobody using the toilet. A few drops of food colouring in the cistern that appear in the pan without flushing confirms it.

Either way it wastes water all day, which shows up on a metered bill. Our guide to a toilet running constantly covers how to tell which valve is at fault and how each is fixed.

Gurgling in the pan

Gurgling or bubbling in the pan after flushing isn't coming from the cistern. It means air is being pushed or pulled through the water in the pan because it can't move freely through the drain. The usual causes are a partial blockage further down or a blocked soil stack vent or air admittance valve. If the gurgling happens when the bath or washing machine drains too, the vent is the more likely cause. See our guides to a gurgling toilet and a low water level in the toilet bowl, which often goes with it.

When to call a plumber

Call a plumber if a noisy fill valve needs replacing and you'd rather not do it yourself, if the cistern is concealed behind a panel, if the pipes bang every time the toilet or a tap shuts off, if the toilet keeps running or topping itself up, or if the pan gurgles and other drains are slow. Noise that's new and getting louder is worth dealing with before the valve fails completely.

We charge £95 an hour for labour, plus a £60 call-out that's waived when the job runs over 2 hours. Parts are extra and we confirm the price before any work starts. Our toilet cistern repair service covers fill valves and flush valves across London, and the pricing page explains how jobs are charged.

Frequently Asked Questions