Have you done anything that might have uncovered something that could have caused the problem? You mentioned that you have never smelled the smell before, have you lived in the house through a couple of spring rainy season? Have you been running and AC that may be draining water you are not aware of? Maybe you uncovered an old pipe that wasn't sealed correctly? I'm just fishing for something that has changed to cause the smell.
If the outdoor humidity is low, you might try venting out the entire basement and then rerunning the humidifiers for a week or so.