A toilet making a high-pitched squeal or whistle during the refill cycle is typically caused by restrictive water pressure, a worn-out rubber diaphragm seal inside the fill valve, or mineral scale sediment trapped within the internal valve body. As water rushes through the tight valve gap during the refill cycle, aging rubber vibrates at high frequency, producing that distinct noise. Verification: Listen to the refill cycle after servicing or replacing the fill valve cap assembly to confirm the squeal has stopped.
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