Our house has had a weird - not quite sewer, not quite mold - odor that seemed to increase when water was run. We discovered a few months back the drain from the washer had broken so was emptying directly into the crawl space, probably for several months. We stopped using it and spread hydrated-lime all over the crawl space and within about a week the smell disappeared. Then, about a week ago, the smell returned. Still don't know why BUT we took action to repair the known break.
Harry crawl under but found nothing . . . SO we cut a hole in the bathroom floor! We did locate the leak, in a very awkward place but removed the broken fitting, replaced and rerouted the pipes to a bit more convenient location and replaced the floor. Now we wait and see if there is another break we couldn't locate . . . Yep, it's Harry UNDER the floor of the bathroom :)