Why does my house smell musty?

That characteristic musty odor is produced by mold and mildew as they grow, so smelling it means moisture is sitting somewhere long enough to feed microbial growth. Air freshener treats the symptom and hides the diagnostic signal. The places to look, roughly in order of likelihood: basements and crawlspaces, under sinks where a slow drip goes unnoticed, behind washing machines and dishwashers, the HVAC drain pan and evaporator coil, bathrooms with weak or unused exhaust fans, and any spot where an old leak dried but left contaminated material behind.

Indoor humidity is the underlying lever. Keeping it between roughly 30 and 50 percent starves mold of what it needs, and a cheap hygrometer tells you where you stand. A dehumidifier in a damp basement, running bathroom fans for 20 minutes after showers, fixing gutters and grading so water drains away from the foundation, and sealing crawlspaces all attack the cause. If the smell is strong, spreads across rooms, or comes with visible growth over about ten square feet, that is the point to bring in a professional rather than a spray bottle.