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You either have a partial obstruction of the main vent stack or an obstruction in the main drain line. Gurgling is caused by either suction from an obstructed vent stack or by back-pressure from an obstructed main drain.Stick a high pressure garden hose in the main vent stack, up on the roof, and turn it on full to clear the vent stack. If that does not work you will need to use a snake to clear any obstructions in the main or branch drain line.
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