It’s strange that it has a 90 for the clean out and also has an open connection on the right. If this is a vent stack it should be stinking up the place always. I guess maybe when the washing machine drains it creates some negative pressure that causes methane to leak in. If the vertical pipe is actually a vent stack you should be able to plug the open connection on the right side. Another comment is make sure any p trap has a bit of water in it. Traps will also leak methane when they start to dry out.
