Toilet tank leaking can cause a very irritating hissing sound which continues long after the toilet has been flushed. If you hear a noise like that coming from your toilet tank even when it is not being used, your toilet has a leak somewhere. If the water from the tank is trickling into the bowl there is probably an issue with the flush valve.
Step 1 - Lid
Take the lid off the toilet tank so you can see inside.
Step 2 - Water
Before you proceed further, locate the isolator valve below your toilet pipe and turn off the water. Flush the toilet to expel the water which is currently inside the tank. Having turned off the water the tank will not refill.
Step 3 - Flush Valve
Watch carefully as the water descends through the flush valve as you flush the toilet. The flush pipe below it is the pipe where the water flows back into the bowl. On top of that you should see a rubber ring and a valve which fits into the hole at the bottom of the tank. Remove the flush valve and check the rubber washer. It may have become shrunken, which is partly what can cause the leak. Remove it and attach the new valve.
Step 4 - Replace the Valve Washer
Replace the old rubber washer with a new one and secure it in place. Plug the flush valve back into the hole and make sure they fit together neatly. The valve should still remain loose enough that it will lift when you pull the flush lever.
Step 5 - Water Supply
Turn the water supply back on and test the toilet.