Quick Answer:
If the sink stops draining and then makes a gurgling noise, it usually means air is moving through the trap because of a partial blockage or a venting problem. First, stop using the sink, listen for the gurgle, and then check the trap seal level after the stoppage to help decide what to try next.
Why This Happens
Gurgling is the sound of air moving through water in the drain. Two common causes are:
- Partial blockage downstream that traps air and forces it back through the trap as water tries to move past the blockage.
- Poor venting (blocked roof vent or blocked branch vent) so the system pulls air through the trap when water is drained, which can lower the trap’s water level and allow sewer air to move.
Either situation can make the trap lose its seal or sound like it’s being siphoned even if the trap itself isn’t leaking.
Step-by-Step What to Do
Stop using the sink and listen
As soon as the sink stops draining and you hear the gurgle, stop running any water. Stand by and listen: does the gurgle happen immediately after the last drop goes down, or only when another fixture runs? That timing helps narrow whether it’s a local clog or an air/vent issue.
Check the trap seal level after the stoppage
Put a bucket under the P-trap, loosen the slip nuts, and remove the trap to inspect it. Note the water level inside the trap before you remove it if possible. If the trap is unusually low or empty after a drain event, the trap was likely being siphoned. If the trap is full and clear, the problem is more likely a downstream blockage or a venting issue.
- Wear gloves and have rags ready—expect some water in the trap.
- Inspect the trap for hair and debris and clean it out before reassembling.
Try a safe local clear
With the trap clean and reinstalled, try plunging the sink (cover the overflow) to remove a moderate blockage. Avoid chemical drain cleaners because they can damage pipes and are hazardous. If you have a hand auger and are comfortable using it, you can feed it past the trap into the drain to break up clogs.
Check other fixtures and vents
Run nearby fixtures (toilet, other sink) and watch whether the gurgle or stoppage repeats. If the issue appears when another fixture runs, see Drain stops when second fixture runs for related causes and checks. Also consider whether outdoor vents could be blocked—leaves, bird nests, or ice can block the roof vent.
Test and monitor
After cleaning and reassembling, run several gallons of water to confirm the drain flows smoothly and listen for gurgles. If the sound returns, note exactly when it happens (after filling the sink, after another fixture runs, only after heavy flow) to share with a pro if needed.
What Not to Do
- Do not dismiss gurgling as normal if draining stops afterward. It’s a sign something is not working right.
- Do not pour chemical drain cleaners into a trap you plan to remove—you can be exposed to corrosive chemicals and damage pipes.
- Do not keep using the sink heavily if sewage odor or repeated gurgling appears; that can make a problem worse.
When to Call a Professional
- When plunging and trap cleaning don’t stop the gurgling or the stoppage returns quickly.
- If multiple fixtures show the same symptoms (gurgling, slow drains, sewer smell).
- If the issue began after recent work or a repair—check details and consider contacting the installer; see Sink stops draining after repair for situations that commonly follow repairs.
- If you smell sewer gas, see visible leaks, or are uncomfortable removing the trap or accessing roof vents.
Safety Notes
- Wear gloves and eye protection when working on drains. Keep a bucket and rags handy.
- Avoid chemical drain cleaners; they can burn skin and damage pipes. Use mechanical cleaning methods first.
- Don’t climb on the roof to check vents unless you have the right ladder, fall protection, and experience. Consider hiring a pro for vent inspection and clearing.
Common Homeowner Questions
- Why does the sink gurgle only sometimes? — Intermittent gurgles usually mean a partial clog or venting issue that shows up under certain flow conditions.
- Will pouring boiling water fix it? — Boiling water can help dissolve grease but won’t clear hair or solid debris and won’t fix vent problems.
- Can I remove the trap myself? — Yes, many homeowners can remove and clean a trap safely, but if you’re unsure or the trap is corroded, call a plumber.