A toilet squealing when filling with water is usually caused by a worn-out rubber diaphragm seal inside the fill valve cap or high water pressure vibrating against a restricted valve opening. As the water rushes past aging or calcified rubber internal parts during the refill process, it causes high-frequency acoustic vibrations that sound like a squeal or whistle. Verification: Replace the fill valve cap assembly (such as a standard FluidMaster cap kit) and listen to the subsequent refill cycle.
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