A gurgling sink usually means air is being pulled through the trap water because the drain is partly blocked or poorly vented. What happens at other fixtures helps locate the problem.
Notice whether it gurgles during or after draining
Noise during slow draining suggests a nearby restriction. Gurgling when another fixture runs can point to a shared branch or venting issue.
Clean the stopper and accessible trap first
Hair, grease, or soap can narrow the drain and force air to bubble through. Do not remove the trap if chemical drain cleaner is present.
Check nearby drains
If another nearby sink, shower, or toilet is also slow, stop treating the sink as an isolated problem.
Persistent gurgling may require vent diagnosis
The drain system needs air to keep traps stable. If traps lose water, odors appear, or fixtures affect one another, a plumber may need to inspect venting and shared piping.