Toilet makes noise after flushing — groaning or whistling
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Most likely cause & what to check
A groaning, whistling or foghorn noise as the toilet cistern refills is almost always caused by the fill valve (ballcock) — typically a worn or vibrating diaphragm washer.
Remove the cistern lid. As the cistern refills, lift the float arm slightly — if the noise stops, the float arm is set too low, allowing high velocity water to vibrate the valve.
Bend the float arm slightly downward to reduce the water entry rate and stop the vibration, or adjust the fill valve's flow restrictor if it has one.
If adjustment does not help, the diaphragm washer inside the fill valve has hardened with age. Shut off the isolation valve, unscrew the top of the fill valve, and replace the small rubber diaphragm washer (£1–£3 from a plumbers' merchant).
If replacing the washer is impractical (older valves may not have spare parts available), replace the entire fill valve for £8–£20.
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