A Kohler toilet squealing loudly after a flush is typically caused by a worn-out diaphragm seal inside the fill valve, high water pressure vibrating against a restricted valve opening, or mineral scale buildup trapped within the internal valve body. As water rushes through the tight valve gap during the refill cycle, the aging rubber diaphragm vibrates at a high frequency, producing that distinct squealing or whistling noise. Verification: Listen to the refill cycle after servicing or replacing the fill valve cap assembly to confirm the squeal has completely stopped.
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