I live in a two story house built in 1978 hooked up on sewer. When the clothes washer is used downstairs there is a sewer smell in the upstairs bathroom. I did not notice the smell was while the washer was used early on. So I have replaced the wax seal under the toilet. All the sinks,toilets , showers drain as they should. What should be my next step of elimination? Thanks in advance for reading my post and any suggestions.