05 Ford Escape Strange Smell

I have a 2005 Ford Escape, 45,000miles. About a month ago I drove 2500 miles across the country; now there is a strange smell coming from the vents. Smells almost like garbage, or as a friend said.. "smells like feet." What is this? I have the cleanest car around, no trash inside the car and is always vacuumed. Any ideas??
Thanks!