A hissing sound coming from a TOTO toilet after a flush usually indicates that the fill valve is failing to shut off completely, allowing a tiny, high-pressure stream of water to continuously bypass the internal seal into the overflow tube. This is typically caused by hard water mineral debris trapped inside the fill valve diaphragm. Shutting off the water supply, removing the top cap of the fill valve, rinsing out accumulated sediment, and reassembling fixes the issue.
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