[LazyPup]

In order for the sewer gases to enter the house there must be a problem with one or more traps on the drainage system.

After a rain the ground is saturated which slows the leaching action in the leach field, resulting in a rise in the tank liquid level. The rise in tank level increases the back pressure on the gases in the tank which then temporarily increases the amount of gasses present in the drain, waste and vent system throughout the house.

I would suspect that a four bedroom house has two or more bathrooms, and as you stated there are only two adults in the household, consequently the second bath lavatory and tub/shower are seldom if ever used. No doubt the water in the traps has evaporated out leaving the drain lines open to the house.

Solution: at least once every couple months run some water in all sinks, lavatories, tub/showers and drains to insure there is an adequate level of water in the traps. If you have a basement with floor drains pour a couple quarts of water into each drain. That would be a sufficient amount to insure the traps are full, and the excess will simply drain away.

After you have run the water in the traps you might elect to pour a small amount (1/4 cup) of liquid cooking oil in all the unused drains. The oil will float on top of the water in the traps and prevent the water from evaporating out.