I have the same problem in my upstairs bathroom, and the washer machine. Only those two places! I don't have a hot water heater. I found when I disconnected the water hoses to the washer that the cold water hose had the smell. I replaced the hose, and the smell went away for a while. I am thinking that maybe the water collects in the hose and sits there for a while (those two water sources aren't used as much as the kitchen for example). I also find that the smell seems to subside when I replace the house filter. I would love to hear any other ideas!