Does it smell when washer is in use. It could be that the washer syphons the floor drain are the trap on washer is syphoned. Next time you run washer take a light and look in floor drain and see if water disapears from trap when water is pumped out, also couls have a cracked pipe,usually the smell goes up so I would think its comming from floor drain, cover up floor drain with something and see if you smell anything. Let me know.
What kind of hookup you have on wash machine, does it have a trap. You could have a cracked pipe are a bad joint, what kind of pipe is it, cast iron lead joints, are nohub, PVC clay are gal. are copper. You could do a smoke test on pipe first. There must be something between coc. Fl. and joist that is bad Let me know.