sewer gas

ok fellas, I really need your help, advice or suggestions. Heres the deal, we moved into our new house 1 month ago today. House has septic system. Everything works great, sinks and toilets all drain fast, no gurgling no bubbling no hiccups at all. The problem arises when we do laundry, after the first load we start to get a sewer gas smell, the more loads we do the worse the smell gets. The smell is only in the basement, the upper 2 floors have no odor at all. We can take showers and run faucets all day long and no odor, odor only when the washer runs.
The laundry room, which is on the first floor, is clear accross the house with its own dedicated vent. the drain line is 2" for 10 feet then goes into a 3" pipe for 15 feet before it dumps into the main. there is a toilet, a vanity and a floor drain in the basement all traps are full and I know the odor is not coming from any drain down there. All the drains are in the floor joist spacing, no pipes in the cement until it exits the house. I can not find it, plumber is baffled. So I turn to you, the experts of the web world for suggestions. I know every inch of the plumbing system, since I did most of it so I'm sure I can answer any questions.
Septic system is 5 years old, pumped and inspected before we started building.