i have a sewer smell that comes thru the furnace vents in the bathroom and kitchen. the kitchen is located near the batroom. sometimes the smell comes thru the bathroom sink or under it. i have a septic system, was cleaned out in 2000.
The only way the smell can come through the vents is to be drawn into the intake of the fan section,which is typically in the basement or crawl space.So the smell is probably from a backed up vent pipe or dry trap.
Find the lowest drain in the house and pour 5-6 gallons of water down the drain.