Why does my toilet cistern whistle or hiss when it fills?

The noise is the valve vibrating. Water passing at speed through a small gap sets the rubber diaphragm oscillating, and the cistern acts as a soundbox. It nearly always gets worse gradually as the diaphragm hardens and the seat scales up.

What you'll need

• A screwdriver
• A replacement diaphragm or fill valve
• Descaler

Step by step

1. Confirm it is the cistern and not the pipework.A whistle that starts and stops exactly with the cistern filling is the valve. A bang or hammer at the end of the fill is water hammer in the pipework, which is a different problem.
2. Try throttling the isolating valve.Turn the supply valve down slightly and refill. If the whistle changes pitch or disappears, it is confirmed as flow noise through the valve. It also fills more slowly.
3. Identify the valve type.An old brass Portsmouth or diaphragm ballvalve with a long arm and a float ball is the noisiest design. A modern vertical plastic fill valve with a cup float is much quieter.
4. Isolate and drain the cistern.Close the isolating valve, flush and hold the lever until it empties.
5. Replace the diaphragm washer.On both old and new valves, a hardened diaphragm is the usual cause. They are inexpensive and take minutes to change.
6. Descale the seat and the nozzle.Scale on the seat gives the water an uneven path, which is what starts the oscillation. Soak the parts and rub the seat clean.
7. Replace the whole valve if it is an old ballvalve.A modern bottom-entry fill valve is quiet by design, cheap, and fits most cisterns. On a persistently noisy old valve it is the sensible answer.
8. Check the float is not fouling.A float rubbing on the cistern wall or on the flush mechanism makes the valve close in steps, which sounds like intermittent hissing.
9. Refill and listen at the end of the cycle.Noise concentrated in the last few seconds as the valve closes is normal on many valves. Noise throughout the fill is not.

Tips

• Bottom-entry valves discharge below the water line and are dramatically quieter than top-entry ones that splash.
• A whistle that changes pitch when you throttle the supply is definitely the valve, not the pipes.
• If several taps whistle too, look at the stopcock rather than each fitting.

Common mistakes

• Leaving the isolating valve throttled as the permanent fix — It quietens the noise by starving the valve, so the cistern takes far longer to fill and the underlying wear carries on. It is a stopgap, not a repair.
• Replacing the whole toilet because of the noise — The noise comes from a part costing a couple of pounds. Nothing about the pan or cistern is involved.

If it doesn't work

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