we have sewer gas smell in the house. we have checked every ...

we have sewer gas smell in the house. we have checked every thing we can think of to find the problem to no avail..there is no leakage in the basement at the main sewer line, or any lines leading to it. the vent stack appears to be clean with no blockages, we ran a snake down it. we have not noticed any leak's up stairs. also the drains are working fine and the toilet flushes fine. that rules out a full septic tank, and a clogged vent stack. we are at our wit's end...