Not only when the washer drains but when water is let out of the sink upstairs the washer drain overflows.I augured the drain and vent.it goes down fast but wants to back up in the washer drain.it only does it when I let water out of the sink.it does not do it when I just turn the faucet on.I thought maybe the vent pipe still might be blocked a little?...it was making a gurgling sound before but after a augured it that stopped.does anyone have any idea what the problem could be?...oh....I am on a sewer line.