Quick Answer:

A gurgling sink when a nearby toilet is flushed usually means air is being pulled through the sink’s trap because the drain system is not venting properly or a partial blockage is changing the air pressure. Start with simple checks like listening for an air pull, running other fixtures, and making sure traps have water. If the roof vent is blocked or several fixtures are affected, call a plumber.

Why This Happens

Your home’s drain system relies on vents and traps to keep sewer gases out and allow water to flow smoothly. When a toilet flushes it pushes a rush of water and air through the shared drain. If the vent is restricted or a pipe is partially clogged, that air can be forced toward the sink and pull water out of the sink trap briefly. That sudden movement makes bubbling or gurgling noises. A dry or low trap seal (not enough water in the U-bend) can make the sound worse or let odors through.

Step-by-Step What to Do

1. Listen for air pull

• Stand near the sink and place your ear close to the drain. Have someone flush the toilet while you listen.
• If you hear a sucking or strong bubbling noise from the drain, that’s a sign air is being drawn through the trap.

2. Run water in other fixtures

• Flush other toilets and run water in the tub, shower, and other sinks. Note whether the gurgling happens only at that sink or in multiple fixtures.
• If multiple fixtures gurgle or water backs up, the issue is likely further down the shared drain or the vent system.

3. Fill dry traps

• Pour a few cups of water down the sink to be sure the trap has a full water seal. A dry trap can let air and odors through and cause gurgling.
• Do the same for rarely used fixtures (floor drains, guest sinks) so all traps have water.

4. Check vent airflow at the roof (carefully)

• If you’re comfortable working safely on a stable ladder and the roof is low and dry, a quick visual check can reveal obvious blockages like leaves, nests, or debris in the vent opening.
• From the roof, you can cup your hand over the vent to feel for air movement while someone flushes a toilet inside. Limited airflow suggests a partial vent blockage.
• If you’re unsure about roof work, hire a pro—don’t risk falling.

5. Re-test and observe

• After these checks, flush the toilet again and see whether the gurgling continues.
• If the problem is intermittent, keep a short log (time, which fixture was used, weather conditions) to help diagnosis.

If the sink is slow to drain in addition to gurgling, see Bathroom Sink Fills Then Drains Slowly. If you notice odors after traps sit unused, see Bathroom Sink Smells Like Sewer Gas After Sitting.

What Not to Do

• Do not snake the vent blindly. Forcing a snake into a roof vent can damage vent pipes or push a blockage further down.
• Do not use drain chemicals to fix a suspected vent or sewer problem. These products won’t reach roof vents and can be harmful to pipes and the environment.
• Do not climb onto a steep or slippery roof without proper safety equipment and experience.

When to Call a Professional

• Gurgling affects multiple fixtures or you see water backing up into sinks, tubs, or toilets.
• There are recurring sewer odors, sewage backup, or visible leaks.
• The roof vent appears blocked and you’re not comfortable removing debris safely—or removing it doesn’t stop the gurgling.
• After basic steps the problem returns or you can’t locate the source.

Safety Notes

• Use caution on ladders and roofs: work only on dry, stable surfaces and have someone hold the ladder.
• Wear gloves and eye protection when inspecting drains or traps. Avoid putting your hands into drain openings.
• Avoid chemical drain cleaners; they can burn skin, damage pipes, and make a professional’s job harder.

Common Homeowner Questions

• Why does the sink only gurgle when the toilet is flushed? The sink and toilet share a drain or vent; the toilet flush changes air pressure and the sink trap reacts when venting or flow is restricted.
• Can I clear a vent blockage myself? You can do a visual check and remove loose debris if it’s safe, but don’t attempt roof work you’re not trained for—call a professional instead.
• Will pouring water in the sink fix it permanently? Filling the trap may stop gurgling caused by a dry trap, but it won’t fix vent blockages or downstream clogs.