I put a modular home on a basement this past summer. The Septic system is composed of a 1200 gal concrete tank that drains openly in a pasture about 300 ft from the house. Late this summer we began to get a strong sewer smell in our basement(right after a 5" rain). The footing perimeter drain was connected on the down side of the septic tank to an 8" drain tile. The smell seemed to be coming from the rough in box under the basement tub(from under the slab). I have since sealed this area with concrete. It seems to be worse when the washing machine is running and we average 4 or 5 a day. I have read several posts and suspect I have a vent or back pressure problem that is forcing gas past the water in the traps - all have water in them. There is a mechanical vent in the wall downstairs for the toilet. I believe all tub, lav, and floor drains are ahead of the stool outlet. The original house plumbing was not modified and drops straight down into the under slab sewer drain. Should there have been a vent ran from the downstairs through the roof?

. Smells and noises are the most difficult to track down if you're not there.
While we can't pinpoint the source we can point you to the most likely offenders.
Since you're using the pasture as a drain field the effluent might have peculated back underv the house and come up in the tub dap out.

"I have read several posts and suspect I have a vent or back pressure problem that is forcing gas past the water in the traps"
This could very well be the problem. Regards, Tom