Toilet Making Hissing Sound: Causes and Fixes

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Is your toilet making hissing sound? I know this can be quite frustrating, especially if you have no plumbing knowledge. Do you wish you had a quiet flush toilet?

In most cases, the reason why your toilet is making a hissing sound, whistling sound, or air noise sound could be a sign that the fill valve doesn’t close and seal properly. This causes water or air to leak from the fill valve.

Toilet Hissing and Not Flushing

It is normal for a toilet to make noises when flushing. When no one is using the toilet, though, it should be silent. Therefore, if the toilet produces hissing noises when it should be quiet, it might indicate a plumbing issue.

Due to the flapper chain, the toilet may hiss and fail to flush. Check if it is too tight since it may prevent the valve from closing. You can shake the chain to check whether it has slack. If the flapper chain is too tight, you should loosen it.

Why is My Toilet Hissing and Leaking?

Occasionally, toilets emit odd sounds such as screaming, gurgling, or hissing. These sounds are primarily associated with air or water movement.

If the toilet emits a hissing sound, it could be that the fill valve is leaking water or air. Regardless, you will notice toilet leaks if the valve does not properly close. Now, let’s talk about how to remedy your toilet hissing problem.

Understanding How a Toilet Works

At first glance, toilets appear relatively simple: a waste pipe running thru the floor and a reservoir of water (known as a cistern) waiting to be flushed into it when a button, lever, or chain is pressed or pulled.

Most systems are essentially mechanical: gravity empties the cistern and washes the bowl clean for reuse when the chain is pulled. They are mechanical in the sense that they flush & refill using internal levers, which are examples of what experts refer to as simple machines.

a GIF image showing how a flush toilet works

There is a bit more to restrooms than this. The cistern must automatically refill from a side-mounted faucet when a toilet is flushed. The refilling process must take long enough to refill the cistern without causing it to overflow.

Additionally, the “hole” in the toilet bowl is more complicated than it appears. You may have seen that toilets always retain a small amount of water even after being flushed. Some water is constantly caught in the S-bend, a sizeable curved conduit at the base of the toilet (or S-trap).

This small amount of water efficiently closes up the sewage line beneath it, preventing bacteria and foul odors from entering the bathroom. In addition, the S-bend causes the pipe leading from the toilet bowl to curve upward before bending back down.

This implies that when water enters the bowl from the tank and empties through the pipe, it has sufficient force to create a siphon (sucking) action, effectively draining the bowl.

What Happens When You Flush?

To flush the toilet, press the handle, which activates a lever within the cistern.

The lever releases a valve known as the flapper (green), allowing the cistern to drain into the bowl below through a siphoning process.

Water from the cistern runs through perforations in the rim, washing the bowl and flushing away the contents.

There is sufficient water descending from the tank to flush your toilet at the S-bend (S-trap). This creates a vacuum effect that cleans the bowl. Additionally, it guarantees that some water stays at the bottom of the bowl, which fosters hygiene.

The toilet’s contents are drained into the main drain.

mechanisms of how flush toilet works

The red plastic float lowers as the tank empties, causing a lever to tilt.

The lever tilts to open the ball valve at the cistern’s base (or on one side), which functions much like a faucet (tap). Pressurized water enters the cistern, replenishing it and forcing the float back up. When the float reaches the appropriate level, the ball valve shuts off the water flow, and the toilet is ready for another flush.

How to Fix Toilet Making Hissing Sound

Clean the Inlet Valve & Seal

If you suspect that debris or sediments from your water supply are clogging the fill valve, flushing it out will fix the problem, allowing your tank to fill again quietly.

This task should not take more than 15 minutes of your time.

You will need a screwdriver, a plastic drinking cup, and follow the steps mentioned below:

  • Turn off the water supply as mentioned above (simply locate the hose that connects to the bottom of the cistern and turn it to the “OFF” position.
  • Then flush the toilet to drain the water from the tank.
  • Find the inlet valve in the vertical assembly just above where the water supply hose connects to the tank. Even though the tank components vary by design, the cap is, in most cases, a different color and can easily be removed by simply pressing it down and twisting it the way you do with a childproof bottle cap. Alternatively, you can remove one or two screws using a screwdriver.
  • Then remove the seal (a plastic disk or rubber disk) located underside of the cap. Pry it using a small flat-head screwdriver or simply with your fingernails. You can rinse it on your faucet to remove accumulated dirt and debris.
  • Invert the plastic drinking cup over the top of the inlet valve and hold it there. Turn the water back on for a couple of seconds- the cup stops the water from spraying out.
  • While you hold the plastic drinking cup in place, turn the water supply on for about 10 seconds. The water will flush out any sediments or debris trapped inside the valve.
  • Inspect the seal you just cleaned from tears, rips, or warping. If it’s in good shape, put it back in the cap and re-attach the cap to the inlet valve. If the seal is damaged, here’s what you should do.

Replacing Inlet Valve Assembly

If replacing the worn-out seal didn’t fix the problem, it could be the entire valve assembly is faulty.

Most likely, it could be hard water deposits that form within the lower end of the inlet valve assembly. You can only solve this problem by replacing the entire inlet valve assembly.

This activity is a bit technical, and I would highly recommend you call a qualified plumber to help.

However, if you’re confident, you can fix it easily. However, you may want to buy two replacement valves just in case you break the first one.

These valves cost approximately $20-$35 x2, which is relatively cheaper compared to the cost of a plumber.

If you go the DIY way, you will need an adjustable crescent wrench, water-pump pliers, and the whole activity will take less than 3 hours.

To replace the entire inlet valve, you will need to remove the vertical valve assembly from the cistern, disengage it from the flush lever and from the arm responsible for lifting the flapper.

Since the inlet valve varies, you will need to confirm with the manufacturer how it should be removed. Check the manufacturer’s website.

After removing the entire inlet valve assembly, take it to the nearest home improvement store and get a replacement that matches your previous one. The replacement kit contains everything, including nuts, washers, and seals.

The replacement kit also contains information on how to install and adjust the height of the cistern.

Inspect and Repair the Refill Valve

When a toilet is flushed, clean water empties from the cistern into the toilet bowl, and the cistern is filled with fresh water from the home supply. The fill valve is responsible for ensuring that the water level is correct.

The fill valve may be obstructed by silt deposit; it could be improperly calibrated or fail with time, causing the tank to overfill. The hissing noise might be caused by excess water running through the overflow pipe.

Your best remedy for this problem would be to recalibrate the valve if it was incorrectly calibrated or replace it if it’s damaged.

The most common fill valve type is the ball-cock, also known as float valve, and resembles a little inflated balloon tied to a rod. It regulates the flow of water.

Find the connection between the float and the rest of the system, which typically looks like a pair of screws or plastic clips, to adjust the water level. Adjusting the float to rest underneath the overflow pipe should resolve the problem.

Inspect the Flap Valve

If you tried the fill valve repair suggested above and it did not solve the issue, the flap valve could be the source of the problem. The flap valve is often made of black, gray, or blue hard plastic, and its purpose is to raise when the toilet is flushed to allow water to pour into the bowl.

The seal is housed on the bottom of the flap valve, which controls the water flow while the toilet isn’t in use and might become brittle or clogged with debris over time. Small quantities of water that seep into the bowl might result in a loud hissing sound.

Regular maintenance of the flap valve & seal is adequate to resolve the issue. You will need to turn off the water supply, then unscrew the flap valve. 

Examine the seal, then clean it with a sponge. If you notice that the valve seal appears to be broken or damaged in any manner, it must be replaced. Take the seal at your local home improvement store to see if they got a matching replacement for you.

Final thoughts

Toilet making hissing sound is a common problem that most homeowners experience. The hissing sound is not only annoying, but the continuous water flow can significantly contribute to increased water bills.

As I have mentioned above, fixing this problem is a simple thing to achieve using readily available tools in your home. Alternatively, you can call in a professional plumber to fix the problem for you.


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