Sewer gases coming up from showers on one side of house and through the washer vent on the other. house is 3 years old, two story, two vent pipes on roof on respective sides. I've seen sureseal ads, have poured water into accessible traps. Would positive ventilation be a solution?
There mite be a clogged vent. The trap is getting siphoned out when the drain sucking for air. if it start to smell then but some water down the drain to stop the smell instill you get a chance to run a snake down the vent.