When a sink drain gurgles, it means that air is being forced upward through the standing water in your P-trap due to a pressure differential within your plumbing system. This typically indicates either a partial blockage in the branch drain line—where rushing wastewater traps and compresses air pockets—or a restricted roof vent stack that prevents smooth atmospheric airflow, creating a vacuum that pulls or pushes air bubbles through your fixture traps. Verification: Check whether other plumbing fixtures gurgle when simultaneous water is discharged.
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