Your toilet is one of the most vital devices in your home. It's also one you may take for granted until it starts making funny noises. One of these noises is an annoying hiss that could be keeping you awake in the wee hours of the morning. This hiss is the sound of water refilling your toilet tank and always happens after you flush, but prolonged or intermittent hissing could be the result of either a faulty flapper or a maladjusted fill valve.
Basic Toilet Mechanics
It is first necessary to understand how a toilet works before attempting to determine the cause of the hissing noise. The toilet fills and flushes with the fill valve and flapper. The fill valve is typically on the left of your toilet tank reservoir and looks like a tube connected to a black floater. It fills the tank with water until the floater reaches a certain level and then stops. The flapper is a rubber stopper at the bottom of your reservoir. The flapper lifts up like a trapdoor when you push down on the toilet handle, flushing out the water.
The flapper is typically made of rubber that tends to degrade over time. You may notice it start to crack or split along the edges, which could prevent it from making a water-tight seal around the valve at the bottom of the tank. Water will continually seep into the toilet bowl when this happens, even when you haven't flushed it, and the fill valve will continually replace the lost water to keep up, which leads to the constant hissing.
Broken Fill Valve
The fill valve may either be broken or maladjusted. The floater essentially flips a switch in the valve's tube, which allows water to flow into the toilet tank when it drops below a certain level. You can typically see the waterline on the interior of the toilet tank, where the walls sharply turn from dark to light. The valve may be broken, clogged, or need adjusting, which will lead to more water flowing in even when there doesn't need to be. This is what creates the hissing sound.
Obviously you must figure out which item is causing the problem before you consider replacement. Allow the tank to fill with water, and shut off the water supply by reaching down behind the toilet and shutting off the water valve. Return a few hours later and examine the tank's water level. You have a broken flapper if it is significantly lower than before or has drained completely. A broken fill valve may be diagnosed by observing the overflow tube, which juts out vertically a few inches above the water line and catches overflowing water before it can spill out of the tank. You must replace the fill valve if water is constantly flowing into this tube.