In certain wind conditions the drainage system has been getting pressurised slightly via the vent stack for the Biorock sewage treatment plant’s primary tank and lifting the Air Admittance Valves inside the house. The fix for this is to fit a wind-driven rotating ventilator cowl to the vent stack, which ensures that the drainage system stays under negative pressure whatever the wind direction.

More info in an update at the end of the Post for the Biorock unit.