Some blame the Everett water treatment facility, and others blame the Cedar Grove composting plant.
To get to the bottom of this smelly mystery, city officials are trying to trace where exactly the stench is coming from using small devices called e-noses.
"These devices are an emerging technology and we're using them because they're the best technology to use," said Craig Kenworthy with the Puget Sound Clean Air Agency, who is conducting an air-stench study in Everett and Marysville.
From now until November they'll use e-noses -- and professional sniffers too.
"They had to go through passing a smell test," Kenworthy said.
Cedar Grove is facing two lawsuits from neighborhoods over possible stenches from their facility. While a company spokesperson says they can't talk about the litigation, she did say since human smell is subjective they're hoping technology will prove they're not the one that smells.
"When you combine it with meteorology you can say, OK, these chemicals were coming across the e-nose and the wind and the meteorology would indicate that that was coming from this particular source," Kenworthy said.
Officials say the smells are not harmful, just stinky. And in the end, Kenworthy says they're confident they'll pin-point the culprit.
The results of their sniff test should be released in November.