A whistling FluidMaster toilet fill valve is typically caused by restrictive water pressure, mineral scale buildup inside the valve body, or a partially closed water supply shutoff valve. To fix this, first fully open the wall shutoff valve behind the toilet to ensure unrestricted water flow. If the whistling persists, snap off the top cap of the FluidMaster valve (while lifting the float arm), inspect the internal rubber seal for debris or wear, flush out mineral sediment by turning the water on briefly with a cup over the valve, and snap the cap back on. Verification: Listen to the refill cycle to confirm the high-pitched whistle has stopped completely.
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