Hi we have had our house 11 after replumbing and never had a smell problem from the washing machine drain. We do not think there is a p trap. The upper level bathroom of our home is not used much now. The washing machine drain on our lower smells like sewer especially when the upper level bathroom is used. After 11 years this is strange. We have turned all the sinks, toilets and showers on in the whole house and it still smells any ideas?