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Sounds like inadequate venting for the drain lines. Could have been build this way or maybe a clog has developed in the vent pipe. Such as birds or squrrels building a nest in the vent pipe up on the roof. You should hope that it's a clog, which can easily be removed. In order for proper vent pipes to be installed the walls will need to be opened up.
Dennis D. Gehman, MCR - Master Certified Remodeler & Master Registered Plumber
Gehman Design Remodeling
Harleysville, PA 19438
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