[Solved!] Sink Gurgles When Toilet Flushes? Here’s Why & How to Fix in 30 Minutes

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Picture this:

You’re washing your hands at the kitchen sink. Meanwhile, someone flushes the toilet, and your kitchen sink starts gurgling.

You aren’t sure if this air bubble noise issue is severe enough to warrant spending $70 to $120 per hour (the average cost of a plumber before trip fees and materials)

This issue is usually caused by a blocked drain line or a vent pipe. Sometimes, it can indicate a damaged sewer main line or a P-trap air admittance valve issue—leading to toxic gases back-up or water overflowing from the toilet.

But how do I differentiate? And more importantly, how do I fix it without calling a plumber? What if my home has a septic tank, not a city sewer line?

Not an issue! I’ll share everything I’ve learned from several licensed plumbers while running a full-service construction company in Phoenix, AZ.

Let’s dive right into it!

Understanding Wet Vented Plumbing Systems

An overview of Drainage, Waste, and Vent (DWV) System

A wet vent is a pipe that doubles as a waste pipe and a vent simultaneously. These days most bathrooms are wet-vented, saving a lot of time and money due to the less piping required. For example, Kurt says he uses his lavatory as a wet vent for the shower/tub about 90% of the time. “It’s only when the conditions seem unrealistic that I dry went a shower,” he adds. 

If the drains are close (like your sink and toilet drain), they are usually vented through the same drain mainline and the vent pipe. If there’s a clog in the drain pipe or the vent pipe, using a plumbing fixture (like flushing the toilet) leads to air being forced into or out of another fixture (like the sink.) This clog and air force cause air bubbling in your sink when you flush the toilet. 

How to Diagnose the Problem and Narrow Down to the Solution

The most important question is to ask:

Does the gurgling affect only a particular sink or multiple plumbing fixtures? 

A specific sink drain is clogged if the blockage affects a particular toilet or shower.

If you don’t find any clogs, it’s possible that the Air Admittance Valve (AAV, if you have one) or the P-trap needs to be replaced.

If your home has a sewer system:

If all the sinks or multiple plumbing fixtures (like the bathtub or the washing machine) in your home gurgle, this could indicate a main sewer line block. You might also experience water draining slowly or water back-ups in the shower, bathtub, or sink. 

If your home has a Septic system:

You could experience backups if your septic tank is full. The United States Environment Protection Agency (EPA) recommends emptying the tank every two to three years by mechanical pumping. Also, we have noticed quite a few cases of sink gurgling when something clogs the filter in the septic tank. 

No matter what your situation is: We’ll guide you step-by-step to get help rid of your sink gurgling. You can use the above tips to narrow down the issue and skip to the relevant fixes below.

Before you begin, ensure that the municipal sewer line is not clogged by confirming this issue with your neighbors. If multiple families report this issue, it might be worth calling your city’s municipal sewer authority.

Kurt Joyce

Kurt Joyce

1. The Problem: The P-trap is Clogged 

What is a P-trap: A P-trap is a u-shaped bent waste pipe that connects the sink’s drain to the sewer system or septic tank. You will typically find it underneath your sink. It also stops dangerous sewer gases like hydrogen sulfide and methane from entering your home. 

How to identify if this is your problem: Along with the gurgling noise, you experience water backflow, or the water in the sink takes longer than before to flow down. Also, if your sink smells like rotten eggs, its likely indicates that the P-trap is clogged.

2. The Fix: How to Unclog the P-trap or Replace the Air Admittance Valve

If your P-trap is underground or inside your wall, you will need to call a professional.

Here’s how to effectively clean a P-trap:

  1. Turn off the water faucet and place a bucket directly under the p-trap. 
  2. Hold the connecting pipe and unscrew the p-trap nut using a plier to remove it. 
  3. Depending on how full your P-trap is, some water will spill out. The bucket we placed in step 1 will collect the water. 
  4. Clean the trap using a bottle brush 
  5. After cleaning, line up the pipe correctly and screw the trap into its place.
  6.  Turn on the sink faucet to check for leaks. Tighten the bolts until there are no leaks.  

3. The Problem: The Air Admittance Valve (Auto-vent) is Faulty

How an AAV Works (Picture: Carson Dunlop)

What is an Air Admittance Valve: AKA Studor valve: This one-way mechanical valve installed after the trap help address the negative air pressure when a fixture is drained (like flushing a toilet.) Sometimes, if you can’t (or don’t want to) vent through the roof line, you can use it for proper ventilation and prevent sewage gases from entering your home. It is typically used at an island sink or to prevent roof penetrations.  

How to identify if this is your problem: If your sink drains very slowly, even when you’ve ensured there are no clogs, installing an AAV can help dissipate negative pressure caused by toilet flushing—thus eliminating the loud gurgling sink noises. If you’ve an existing AAV, it may be time to replace it. 

4. The Fix: Install or Replace the Air Admittance Valve (Auto-vent)

While most states allow the use of AAVs, some state and local building rules prohibit its use. Check with your local Authority Having Jurisdiction (AHJ) to find the most up-to-date information. 

Kurt Joyce

Kurt Joyce

Here’s how to install or replace an AAV, also called a cheater valve:

  1. Disconnect and remove the trap bend using pliers.
  2. AAVs are typically placed between the P-trap of a fixture and the drain line. Ensure to install it at least 4″ above the horizontal branch. 
  3. Measure, cut, and chamfer the pipe accordingly.
  4. Install the assembly and attach the drain pipe. 

5. The Problem: The Kitchen Sink or the Toilet Drain is Clogged

Soap scum and hairs often clog a sink drain

What causes a clog in my shower drain? The most common causes of a shower drain clog are human hairs, soap scum, and hard water. Hairs can not only restrict water flow but also stick to grime and soap scum to worsen the problem. The soap can mix with the hard water to form a soap scum that sticks to the inner sides of the drain. If your area has hard water, calcium and iron can stick to the sides of the drain.  

6. How to Unclog the Kitchen Sink Drain

6.1 Easy Fixes: Remove Visible Obstructions Using a Coat Hanger, Hot Water, or a mixture of Baking Soda With Vinegar

Remove the drain cover. Pour hot water around the edges. This can help dissolve the build-up of soap scum. If you’ve PVC pipes, skip this step as the heat can damage PVC pipes. Remove the soap scum and hairs. You can use a flashlight if nothing is visible. 

We hear you asking:

Can I use a wire coat hanger to clean the drain? Yes, you can use a coat hanger if the clog is near the surface of the drain. We’ve seen some plumbers’ advice against it since it can get stuck, worsening the clog. But in our experience, it’s alright to do so as long as you’re careful about it.

Alternatively, you can also try mixing 1⁄3 cup of baking soda with 1⁄3 cup of vinegar and flush it down the drain. Cover the drain and let it sit for 15 to 30 minutes. Pour boiling water down the drain. The bubbling reaction from this chemical reaction can sometimes help break up the grease or oil residues.

6.2. Plunge the Sink 

A plunger is a simple plumbing tool that uses suction and pressure to remove a blockage in the drain. 

Before plunging:

Use a wet rag or duct tape to block the overflow opening on the front wall of the bathtub. To do this, you may need to remove the cover on the overflow tube. 

Here’s why:

If you don’t seal the drains, the air pressure can escape through other drains—making our plunging process less effective. 

How to (correctly) use a plunger to plunge a sink drain:

  1. As mentioned above, ensure you’ve sealed the overflow using a wet rag or duct tape.
  2. Cover the drain opening completely with the plunger. The tighter the seal, the better. Try using a silicone lubricant if you don’t get a good seal. 
  3. Fill enough water in the tub till the plunger cup is covered. 
  4. Thrust the plunger up and down. Repeat this process a few times.
  5. If the water drains away, the unclog has been successful.

We don’t recommend using chemical cleaners because they can cause damage to the plumbing, mix with the water supply, and be a hazard for us and the environment. To top that, they aren’t even very effective in removing clogs. 

Kurt Joyce

Kurt Joyce

6.3. Plunging Doesn’t Work? Try Unclogging With a Drain Snake Auger (Plumbing Snake)

What is a Drain Snake? 

A drain snake is a flexible auger that can help remove clogs when a plunger doesn’t work. The coiled-shaped metal wire can break up obstructions to enable water to flow again.

To unclog a sink drain, you’ll need a hand auger. In our experience, it’s not a good idea to run a hand auger through toilets for two reasons:

  • Toilets often have a larger drain pipe than most plumbing fixtures—leading to a tangled snake. Most plumbers recommend using a 1/4″ wire for less than 2 inches of the drain pipe. 
  • The wire can damage your porcelain bowl. 

To unclog a toilet drain, use a closet/toilet auger. The hook-shaped metal tubing makes it easier to insert the auger into the toilet. 

How to Unclog With a Drain Snake or a Closet Auger

Most augers can reach up to 25 feet—so you can easily find one for your needs. Crank the auger until you experience some form of resistance. Gently push the auger back and forth a few times to break down the clog. Then, remove the auger and run the water for about 5 minutes to flush the clog. 


7. The Problem: The Vent Pipe/Stack is Blocked

What is the vent stack? The vent pipe is a vertical pipe located on your home’s roof. Its advantages are two-fold:

  • It regulates the airflow in your plumbing system. The drain pipes cannot move the waste and water without proper air circulation.
  • It removes sewer gases. Sometimes, these foul and dangerous sewer gases can back to your home. If you can smell strange gases, it can be a tell-tell sign of an issue with the vent pipe.  

What causes the air vent to block?

  • Common things that block the vent pipe are leaves, debris, insects, and sometimes—dead mice and nests (yes, we’ve seen the last two!) 
  • Your sink might gurgle when the toilet is flushed, especially in cold weather, because in colder climates, the water condenses into the ice near the air vent—thus blocking it. 
  • Sometimes, a contractor can forget to remove the caps on the vent pipes after running a pressure test or something.
  • There are structural issues, such as the wrong slope of the horizontal venting system in the attic. 

8. The Fix: Unclog the Blocked Vent Pipe/Stack 

If unclogging the sink doesn’t work, the issue is likely a clogged vent pipe. 

How to clear a blocked vent pipe? 

  1. Clearing a blocked vent pipe will require you to get on the roof—so make sure you’re comfortable doing that. 
  2. If you’ve multiple vent stacks, look for the one near the bathroom where the bubbling or gurgling happens. If you can see only one pipe, multiple vents have been combined into one. 
  3. Remove any leaves or debris you may find at the surface. 
  4. Use a thin flashlight to look further down. If you see any obstructions, such as dead mice or insects, try removing them with a wire hanger. 
  5. Pour down the water in the vent using a garden hose.
  6. Consider installing a screen over your vent pipe. Keeping foreign objects out of your vent pipe prevents costly plumbing repairs. 

9. What to Do if Everything Fails

In our experience, unclogging the sink, vent pipe, or replacing the AAVs should fix the gurgling toilet issue in most cases. In the rare case it does not, here are some additional tips:

  • Your main sewer line may be clogged. A tell-tale sign of this is multiple plumbing fixtures in your home are bubbling, you hear a strange gas smell, or if there’s water back-up. At this point, we recommend calling a professional sewer repair company. They can use hydro-jetting to shoot up to 4,000 PSI of water through the pipes—enough to break down tree roots or anything clogging the sewer line. 
  • If your home is new, it’s possible that the vent has not been installed or is incorrectly installed. If everything fails, it’s time to call a professional licensed plumber. 
  • If your sink gurgles when you use the toilet and your home has a septic tank, ensure that the tank is pumped/emptied every two to three years. Also, try removing the clog in the septic tank’s filters. 

Frequently Asked Questions (FAQs)

Why Does My Sink Gurgle But it Drains Well? 

If you hear loud gurgling noises from the sink, but it still drains well, it may be due to the negative pressure in the system when a toilet is flushed. This can usually be solved by installing or replacing an Air Admittance Valve (AKA Studor or Cheater valve)

Why Does My Sink Gurgle During Heavy Rains?

If your toilet only gurgles when it rains, you might have rainwater pouring into the septic tank— the excess water flow restricts airflow, causing overflowing or gurgling noise. A cracked pipe or a sump pump in the basement can cause this problem. 

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