What is causing the musty smell in my house?

Many things can cause a musty smell. You might need a ventilating fan in the bathroom. You could have poor indoor air quality. If you have a crawl space, then that odor is probably coming from high moisture levels in your crawl space that promote the growth of musty-smelling mold and mildew. A closed crawl space would remedy moisture problems and take care of any musty smells coming from it.