I'm just going to venture a couple of wild guesses. Is it possible a piece of laundry (a sock?) could be lodged under the agitator and is molded? Is the drain functioning properly. It should be 2" pipe and be trapped near the floor. Perhaps there is a blockage in the drain and gray water is getting into the washer. As I said, these are wild guesses, it will be interesting to hear what it actually is. Likely some of the pros will have first hand experience with a problem like this.