Problem
A hissing sound comes from the toilet tank even when the toilet is not being flushed.
Possible Causes
Faulty fill valve
Worn flapper
Water leaking slowly into the bowl
Improper float level
How To Fix It
Remove the toilet tank lid
Check the fill valve for constant water flow
Inspect the flapper for wear
Adjust the float level if needed
When To Call A Professional
If the hissing sound continues after replacing the fill valve or flapper, a plumber may need to inspect the toilet components.