A toilet making a loud foghorn or groaning sound when refilling is usually caused by a vibrating fluid fill valve washer, high municipal water pressure, or a partially closed water supply shutoff valve. To fix this, fully open the wall shutoff valve to ensure unrestricted water flow. If the foghorn sound persists, replace the worn-out rubber diaphragm seal inside the fill valve cap assembly (such as a standard FluidMaster cap replacement kit). Verification: Listen to the tank refill cycle to confirm the deep vibrating noise has ceased completely.
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