Quick Answer:
Gurgling when a sink drains usually means air is moving through the drain because the trap seal is being disturbed or the drain venting is poor. To check, drain a full sink and listen for the gurgle; then check if the trap seal level drops afterward. If the trap is losing its water seal, do not ignore it—fixing the trap or venting is needed to stop possible sewer gas and ongoing siphon behavior.
Why This Happens
A trap (P-trap) holds a small amount of water to block sewer gas. Gurgling happens when that water seal is pushed or pulled so air moves past it instead of staying in place. Common causes:
- Partial blockage in the trap or further down the pipe that causes air to push back through the trap as water moves.
- Improper trap configuration (an S-trap or a trap arm that drops too steeply) that allows siphoning during fast drains.
- Blocked or restricted venting, so air cannot enter the system smoothly and instead pulls through the trap.
- Recent work on the drain that left joints loose. If you recently worked under the sink, check **Trap leaks after installation**.
- Hair and debris catching in the trap and slowing flow; for those cases see **Trap blocks hair flow**.
Step-by-Step What to Do
Step 1 — Reproduce and listen
- Fill the sink to normal level (or fully) and then release the stopper so it empties quickly.
- Listen at the sink, the nearby drain, and other fixtures for gurgling sounds while it drains and just after.
- Note exactly when the gurgle happens (during flow, as it finishes, or after). That timing helps narrow the cause.
Step 2 — Check the trap seal
- Place a bucket under the P-trap. Loosen the trap’s slip nuts and remove the trap carefully to see how much water remains.
- If the trap is empty after a drain or the water level is much lower than expected, the seal has been disturbed or siphoned.
- Reassemble the trap securely after inspection. If unsure how to re-seal slip joints, keep the parts and call a plumber.
Step 3 — Inspect for blockages
- With the trap removed, clear any hair or debris. Check the trap arm and the pipe going into the wall for obvious clogs.
- Use a flashlight or a plumbing snake only as far as the trap arm; do not force a snake into pipes you cannot see—this can cause damage.
Step 4 — Check venting and other fixtures
- Run water in other fixtures (toilet, tub) while listening for gurgles at the sink. If multiple fixtures gurgle, the vent may be blocked.
- A blocked roof vent commonly causes these symptoms. Do not climb on the roof yourself if you are not comfortable—call a professional.
Step 5 — Test again
- Put the trap back, run the full sink test again, and listen. If the trap now holds its water and gurgling is gone, the job is done.
- If gurgling continues or the trap repeatedly loses water, you likely have a venting or configuration problem that needs a plumber.
What Not to Do
- Do not treat gurgling as harmless if the trap seal is being disturbed.
- Don’t pour large amounts of harsh chemical drain cleaners into the sink to “fix” gurgling; they can damage pipes and won’t solve venting or configuration problems.
- Don’t repeatedly run the disposal or force high flows to try to wash the problem away—this can make siphoning or backpressure worse.
When to Call a Professional
- Call a plumber if the trap keeps losing its water seal after cleaning, if gurgling affects multiple fixtures, or if you suspect a blocked roof vent.
- Ask for a diagnosis of trap configuration if an S-trap or incorrect trap arm length may be present—those setups can cause persistent siphoning and should be corrected.
- Also call a pro if you detect sewage smell, or if you are uncomfortable removing the trap or working near roof vents.
Safety Notes
- Turn off any electrical devices near the sink before working under it. Keep the area dry to avoid slips and shocks.
- Wear gloves and eye protection when removing a trap—standing water and debris can be dirty and may contain bacteria.
- Do not climb onto a roof to clear a vent unless you are trained and equipped. Roof work and ladder use carry fall risks—hire a professional if needed.
Common Homeowner Questions
- Why did the gurgle start suddenly?
A sudden gurgle usually means a new blockage or a change in venting; recent work or debris often causes it. - Will pouring boiling water help?
Boiling water can clear light grease but won’t fix siphoning or vent problems and may damage some pipes and seals. - Can I ignore it if there’s no smell?
No—gurgling can mean the trap seal is failing and may eventually allow sewer gas into the home, so check it as described above.