Neither company had any recommendations after an on-site inspection, so this is tough to diagnose over the internet.
Dried out traps are the first culprit. You found one and took care of it. Was it capped properly?
Is the laundry room in the basement or upstairs?
Sometimes the laundry drain hose is run into a drain pipe that is not trapped and the smell eminates from there.
If your washing machine is sitting in a water overflow pan, sometimes the drain in the overflow pan is either not trapped, or the trap is not primed and it too runs dry.
Is the smell just inside, or outside as well? Is it localized...basement/first floor/second floor/bathroom/kitchen?
I'd venture that the traps in little used bathrooms have been inspected and found too be in good shape?
Toilets are in good shape...traps primed, no porcelain cracks, good wax seals.
Did they inspect your vent pipe coming out of the roof to ensure that it was free of obstructions and extended sufficiently above the roofline of the house?
Are you on your own septic? When was the last time the tank was pumped? Leachiing field was found to be in good shape during purchase inspection?
Do you ever hear gurgling or bubbling noises when it's windy outside, or when the washing machine is draining? The noise will usually come from a toilet.
Ask the neighbors if the previous homeowners complained of similar problems. No accusations as of yet, just looking for info.
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