A musty smell is not a nuisance to be covered; it is a measurement you can make with your nose. The odour is produced by fungal growth that is actively metabolising, which means it is telling you that something in the building is wet now. That is why a house smells musty in some weather and not others, why the smell is usually strongest at the lowest point, and why an air freshener makes the problem harder to find rather than smaller.
Does mold have a smell? What the smell actually is
Growing mould releases volatile organic compounds as a by-product of metabolism, and those compounds are what you smell. It follows that a strong musty smell means active growth rather than old damage, and that the smell rises and falls with humidity and temperature because growth does. The question does mold have a smell has a simple answer, yes while it is growing, and the corollary is more useful: a smell that has appeared means conditions have changed, not that a spore has just arrived.
Why the house smells musty after rain, or all of a sudden
Rain raises the moisture load in the ground and the air, and a crawl space or basement that is borderline tips into active growth. Warm humid air entering a cool space condenses on the coolest surfaces, which is why a smelly basement and crawl space odor get worse in summer rather than in winter in much of the country. Running the HVAC redistributes it, which is why the smell can appear throughout the house within minutes of the system starting and why people describe it as sudden.
Black mold smell, and what a nose can and cannot tell you
There is no reliable way to identify a species by odour. What does black mold smell like and what does mildew smell like produce the same honest answer: earthy, damp, like a cellar or old paper, and the descriptions overlap completely. Mildew vs mold smell is a distinction without a practical difference, since mildew is simply thin surface growth. A nose is excellent at telling you that growth is active and roughly where, and useless at telling you what it is.
Finding the source rather than masking it
Work from the smell downward and inward. Check the lowest level first, then the crawl space, then under sinks and behind appliances, then the HVAC return and the ducts, then any external wall that has been wet. Use a moisture meter rather than your hands. If the smell is strongest when the system runs, suspect the ductwork or the plenum. If it is strongest after rain, suspect the ground, the grading or a foundation penetration. Deodorising before the source is found simply removes the instrument you were using.
Questions people ask about musty smell in house
What causes a musty smell in a house?
Active mould growth on something damp, releasing volatile compounds as it metabolises. The smell means the moisture is present now rather than in the past, which is why it comes and goes with the weather.
Why does my house smell musty all of a sudden?
Usually a change in conditions rather than a new problem: rain, a humidity rise, the HVAC starting and redistributing air from a crawl space or duct, or a leak that has just reached something absorbent.
What does black mold smell like?
Earthy and damp, like a cellar. There is no reliable way to identify a species by smell, and the descriptions people give for mould and mildew overlap entirely.
Will an air purifier or deodoriser fix it?
It will reduce what you notice and change nothing about the growth. Worse, it removes the signal you were using to find the source. Find and dry the wet material first.