A toilet making a high-pitched whistling or squealing noise immediately after a flush is typically caused by a worn-out rubber diaphragm seal inside the fill valve, high water pressure vibrating against a restricted valve opening, 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 a high frequency, producing that distinct acoustic 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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