In our basement, our wash machine and a utility sink drain directly into our sump pump. The sink has a trap, but the wash machine does not. The sump pump has a metal grate over it, but there is usually a foul odor coming from the wash machine or the sink area. Is this because there is no trap in the wash machine line?? The piping for this is all behind dry wall, so can't access easy and did not know if you could actually put a trap on a drain line out of a wash machine if I ripped the dry wall out??? Thoughts?? Smells more in the summer than winter.