my bathtub doubles as wash machine drain. i get a very strong sewer gas smell when the machine drains for the fist time each day after 1st load no more smell the tub drains o k and i've opened the trap to make sure water is present it seems o k but i still get the smell once a day.there is no smell from any other fixtures in the bathroom ie. sink,toilet. any advice before i tear up the floor would be helpful