Toilet Making Hissing Sound: Causes and Fixes

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.

The Mechanism of Toilet Flush System?

In order to get a better understanding of how the hissing sound arises, it’s important to have a better understanding of the different parts of the toilet flush mechanisms found inside the flush tank. They include:

Fill valve: the valve opens up to allow fresh water into the flush tank. It features a floater attached to it that descends downwards when the toilet is flushed. As the float goes down, the valve unlocks, and water passes through. When the float floats up, the valve shuts off, cutting the flow of water.

Flush valve: it opens up to release water that, in turn, pushes your poop down the toilet.

Flapper: is connected to the bottom end of the flush valve. After the flash valve closes, it blocks the drain hole to block water from flowing down the tank.

Reasons Toilet Is Making Hissing Sounds

In most cases, the hissing sounds emanate from the fill valve that fails to close completely. It can affect the other parts of the toilet flushing system.

The hissing sound shouldn’t be ignored as doing so could result in increased water bills over time.

When the fill valve is left unchecked, it can block or become defective, and the water will continuously fill the cistern continuously.

If not checked immediately, the amount of water flowing through the tank could add up to hundreds or thousands of liters/gallons over time.

And it’s unfortunate that the water company won’t sympathize with you when they present to you the enormous water bills.

Another reason why your toilet is making a hissing sound could be a faulty float. The float signals the fill valve to shut off or open, and when it’s damaged, it can’t perform its task.

This makes the fill valve fail to shut off, resulting in constant hissing sounds.

How to Fix Toilet Making Hissing Sound

Once you notice the toilet is making hissing sounds, you should immediately shut off the toilet water supply valve. It’s often located on the toilet wall near the toilet base.

Turning the valve to the off position will prevent more water from entering the cistern, allowing you to schedule a perfect time to fix the toilet either by yourself DIY or by calling a professional plumber to inspect and fix the problem.

However, I have found this problem quite easy to fix. You don’t need professional expertise. I’ve successfully fixed the problem myself, and below are different ways you can troubleshoot this problem.

A bit of cleaning the sediments and dirt that settles on the fill valve should solve the problem. You don’t have to replace the toilet.

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 Damaged Seal

If the old seal is damaged, it could be the reason the toilet is making a hissing sound by blocking the valve.

You can get a new seal from the manufacturer or order from Amazon. You can get a variety of seals through this link.

Alternatively, you can take your old seal to your local plumbing store or home improvement store, and they’ll help you find one that matches your old one.

After getting the replacement seal, put it to the cap and reattach it back to the valve assembly.

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.

Final thoughts on Toilet Making Hissing Sound

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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