After having septic pumped out, main line professionally augered and installed a new efficient toilet, water in toilet still bubbles when other drains empty. When laundry water drained, some came up in bathtub. All drains have also been treated with CLR, draining etc. Roof vent has also been checked, and treated for clogs. Septic pump has also been checked, and is working fine.
What could it possibly be now. December has been a very expensive month trying to rectify These problems.