[HelplessHomeOwners]

We have a damp, organic smell in the house. It comes and goes with little rhyme or reason, but tends to be worse at night. It also seems to be coming from under the house and is worse around floor heating vents and where sink drains attach to walls.
We have done some investigating- there do not appear to be any plumbing leaks although there is some standing water under the house (maybe 1/2 an inch) in the deepest corner of the 1 to 2 foot crawl space. We live in a part of Oregon that has a very high water table and the plastic ground cover in the crawlspace has a few holes in it. The grade in the back yard also leans toward the house.
We have a bathroom that shares a wall with the laundry room and occasionally when the washer drains it backs up into the shower basin, usually if we have done a lot of loads of laundry back to back. These rooms are above the corner with the standing water. But we went under the house and ran both the shower and washer at the same time and didn't see any dripping.
We don't know where to start do we call a plumber? Do we replace the ground cover? Do we regrade the yard? Help!