For several months, we have noticed a very unpleasant odor that seems to be coming from the crawl space. We have monthly pest control service check this space and use rodent traps,which they check routinely. The smell is not evident when the furnace blower is on. I found several garter snakes in the space a few months back but believe I got rid of them. Could this be the problem?? Suggestions??

(we live in western WA state)

Thanks for your help.