Locating a Musty Smell in My House?

My house is part of a duplex in a neighborhood that was built in 2005 in north Florida. I moved into it in 2009 and there was no smell. This past May or so (2014), I started noticing a very musty foul stale smell in my kitchen and living room (they're both basically the same space, the kitchen is see-through). The entire downstairs consists of (from the front door) the living room, then the walk-through kitchen, the small hallway with the master bath to the right and the enclosed laundry closet to the left, right through the master bedroom, which leads to the back door.

I can smell the musty stench all throughout the downstairs except for the master bedroom and bath, which is weird.

Earlier on, I could smell it upstairs as well. It's not so bad now, but the stench downstairs is really frustrating! I can't determine the source of the odor. I smell it around the kitchen sink, but then I smell it on the opposite side of the kitchen at the table, then in the living room and near the windows. It's not the laundry closet because I don't smell it in there.

I've always put in plug-ins just to make the house smell more homey. But now I can't because the stench compromises the plug-in and sometimes even overpowers it.

I have no idea what's causing this. It went away for a little while, but now it's back (cue the girl from Poltergeist). It smells like a house that's been locked up for years with no ventilation, but that's not the case, of course. I did get a brand new AC unit last September (2013), which is energy-efficient (apparently meaning that it runs slower and takes it time to get to its temperature or something). But I never had any problems with this stench until this spring, so it can't be that.

Does anybody have any idea what could be causing this? I'm desperate! It drives me to tears sometimes.

By Stacey