A toilet fill valve normally runs after a flush. Hissing or short refills when nobody has used the toilet can mean water is escaping from the tank, or the valve is failing to shut off fully.
Look at the overflow tube
If you can safely remove the tank lid, look for water flowing into the top of the overflow tube. A high water level there points toward the fill mechanism or its adjustment. If the water level falls while the toilet sits idle, tell the plumber how often the refill starts.
Describe the pattern before replacing parts
Record whether the sound starts immediately after filling or several minutes later. A plumber can compare the water level with the overflow and check whether the flush seal leaks. Those observations help identify which part needs attention.
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