Locating a Musty Smell in Spare Room?

We have recently begun to smell a very bad odor in one room of the house. Fortunately, it's a spare room so no one has to sleep in the stink. But we can't find the source of the smell in order to get rid of it. We've moved everything and looked in and under (and over) every item in the room, but still can't find it. We checked the attic above it and the basement below. Also the heater vent.

Nothing. My husband thinks it could be in the wall, but it's not a dead animal (shudder) smell. It's more like laundry that's been left in the washer for a week (admit it, you know that smell). Or maybe a stagnant pond. I know it's a long shot, but does anyone have an idea for locating the smell? In the meantime, we can't even open the door to that room, the reek hits you in the face.

By ~gloria from w. NY