See if you can isolate the smell. Close up a room for a couple of days, then open the door to that room and try to see whether or not the smell is more noticeable than elsewhere. Has this odor been in the house the entire time you've owned it?

Try to follow your nose as much as possible...I would start with the basement floor underneath the bathroom, also under the kitchen sink

Another source for strange smells is sewer gas. If you have floor drains, make sure you keep water in them. Sometimes the water in the trap dries up and allows odors into your house. Also, check to see if your house has a ground level mushroom vent for the entire sewer system...those can smell horrible (that would be outside your house)

Are you doing any renovation where you have uncapped drain lines while you're redoing a bathroom?