A drain making gurgling noises is experiencing air turbulence caused by a partial restriction in the drainage pipe or an obstructed roof vent stack. When debris narrows the interior diameter of a branch pipe, rushing water acts like a piston, pushing compressed air bubbles backward up through the P-trap of the nearest sink or tub. Alternatively, a blocked roof vent creates a negative pressure vacuum that draws air through household water seals. Verification: Inspect the branch pipe or check if other plumbing fixtures gurgle during use.
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