Here is another suggestion....could it be a dead rodent in the walls. Here we have lots of mice and rats who live out in the wild and would like to live in the house. I live in an old house an there is spaces between the inner and outer walls. I put out poison in places I know where they can come in. It is the kind that is supposed to make the rodents seek water and hopefully go out of doors to die. Well every now and then one goes into the wall space and dies there. It makes a musty smell in the house. I don't have a solution except to put out some cedar cuttings to freshen the air and wait a week or too until the smell leaves. It does make my son's Jack Russel dog go nuts though as he searches the perimeter of the rooms looking for the rat.