Why do I hear a hissing sound from my toilet? It's because water or air is moving through your water line into your toilet tank.
Let's talk about water first, which is usually an easy fix once you find the problem. Toilets have what's called a refill valve. It controls how much water goes back into the tank after you flush. If the refill valve is not working properly, you can end up with too little or too much water after each flush.
A hissing sound can mean that the refill valve is still letting a tiny bit of water into the tank because it hasn't shut off like it's designed to. The valve shuts off automatically based on the water level in the tank. A faulty valve or one not set properly will keep running. When you hear the hissing sound, take the lid off your toilet tank. The first thing to check is to see if water is running down the overflow pipe. If it is, your refill valve is not shutting off when the tank is full. That means that water is constantly going down the refill tank and running up your water bill.
How you adjust the water level in the tank depends on the type of refill valve you have. Usually, you will see a screw on older ball type floats to adjust the water level. On newer tower valves, there is usually a marked wheel to turn to set the level. Adjust as necessary until the talk fills, but does not fill high enough to start running down the overflow pipe.
If the water level looks fine and you still hear hissing, check to see if the flow from the refill valve is clear. Just swipe your finger over where you feel the water coming in and remove anything that might be blocking the hole.
If that doesn't work and you still hear hissing, then your problem could be with the water supply to the toilet, a problem further back in your plumbing or even air in the system. All of these possible issues should be investigated by a licensed plumber as soon as possible, because the hissing could mean a leak inside an interior wall. That could lead to serious damage to your home.