Your problem has nothing to do with venting. It is a failure of the septic system. Septic fields which fail due to age are further overloaded by rain saturation, which bring the sewage odor to the surface. The fixture gurgling is the slow drainage as the waste percolates upward slowly to the surface. Pumping of the septic tank should be done immediately. Ask the person pumping the tank to observe the outlet when the tank is pumped. If water flows back from the outlet in any great amount, then either the fields themselves are unusable, or the piping may be clogged. The amount of water returning is an indicator. Hopefully, it is only your tank that needs pumping. The loss of the septic fields could be quite expensive.
