A toilet fill valve becomes loud during refills due to high municipal water pressure, a partially closed wall shutoff valve, worn-out internal rubber seals, or mineral scale buildup restricting water flow. When water forces its way through a tight or damaged valve opening, it generates high-frequency acoustic vibrations that resonate through the ceramic tank and plumbing pipes. Verification: Fully open the wall shutoff valve and test the fill cycle to check if the noise level decreases.
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