As long as I've lived in my house (6 years) whenever the washing machine dumps it water a little water will back up in the shower. It really doesn't back up but a little bit and as soon as the washer stops dumping it goes down. People have told me different things that it could be, partial clog or that there is so much water dumping out of the washer it can't go down the pipe at once so it finds an alternate route until it can drain. The washer and the shower are very close to each other as well about 3 feet. I just want to make sure what's backing up isn't sewage. Wouldn't I know if it was? Wouldn't I be able to smell it? Or is it Just what I think The washer Water?