When our washing machine drains out into the washer box/drain, the water backs up. We know the drain is not clogged as we can run a hose of water through it for some time with no problem. The house has been remodeled, so we do not know if this is new plumbing or original plumbing. It is all sheetrocked in and the drain pipe is hidden or enclosed in a wall box. We can't get under the house to see anything either. We don't know what else to try. The house was built in '66, so maybe there's an old plumbing issue. Any advice?