Is washer old?
If the washer by chance was there when you bought the place, theres probably a reason they left it for you. I have owened washers that smelled like someone crapped behind them. So if it was there when you moved in, move it out. If the washer is new to house, do as majikdragon suggests and see if there is a pee trap at bottom of washer drain pipe. It should look like an upside down horse shoe bend at bottom of drain pipe. If that is not there, that could very well be the problem. Good luck, let us know please...
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