A hissing toilet can be an annoying problem to deal with, but there is more at risk than just an obnoxious sound. Hissing can indicate a water leak. Even if the problem is minor, the hissing means your toilet is wasting water, which can lead to increased usage and costs.
1. Misaligned Flapper
The flapper is inside the toilet tank. It's the flat-ish piece that covers the hole that connects the tank to the toilet below. If it isn't aligned properly, then it won't close fully. Water will continue to leak around the edges and into the toilet bowl which then results in the hissing noise. You can adjust the flapper to realign it and solve the problem.
2. Failed Seal
If the problem persists, then the flapper seal may have failed. In this case, you must replace the old flapper with a new one. You can check for a failed seal with a few drops of food coloring. After adjusting the flapper to the proper position, place a few drops of food coloring in the tank. Wait 15 minutes to an hour, then check the toilet bowl. If the food coloring has leaked into the bowl, then the seal is bad and the flapper needs to be replaced.
3. Toilet Leaks
Hissing could actually be the result of the bowl slowly refilling due to a leak. A crack in the tank causes water to seep out slowly, and the result can be the hissing sound of new water constantly coming in to replace that which is lost through the leak. You can't effectively repair a cracked tank, so it is best to replace the toilet as soon as possible. Otherwise, you will be wasting water and increasing the chances of water damage to the floor and wall behind the toilet.
4. Overflow Tube Issues
The overflow tube is part of the fill valve. This tube prevents the water from overfilling the tank and overflowing. As soon as the water level reaches the top of the tube, it flows in and you will hear a hissing noise. Normally, there is a float attached to the fill valve that stops the tank from filling further long before the water level reaches the top of the overflow tube. You may need to adjust the float if your toilet is hissing due to water in the tube. In other designs, there is no float but the height of the fill valve determines when the water stops filling the tank. In this case, adjust the fill valve so it sits slightly beneath the top of the overflow tube.
Contact a plumbing contractor for more help with your hissing toilet.