An animal make have gotten trapped in a wall if the wall smell like one. Another reason for the smell is mold which occurs when there is water damage.
A "musty" smell would describe a damp animal
damp
Mold typically has a musty and earthy smell, often described as damp or stale. This smell is not usually similar to the smell of chemicals, which can be sharp or acrid. Mold odor is more organic and can be associated with a damp environment.
fart
When bleaching powder (calcium hypochlorite) is exposed to damp air, it reacts with water to release chlorine gas. This characteristic smell of chlorine gas is what you detect when using or opening a container of bleaching powder in damp conditions.
Somewhere that has the damp earthy smell of tunnels and cellars.
Look for water leakage. You might have some water damaged wood.
A damp smell in a small box room could be due to poor ventilation, excess humidity, or water leaks. Mold and mildew thriving in a damp environment could also contribute to the odor. It's important to identify and address the source of the dampness to eliminate the smell effectively.
Spike it up at the back when slightly damp. Do this while your head is upside down!
Put it outside to dry in sunshine and wind
The smell of calcium chloride is generally described as a musty or slightly pungent odor. It is often likened to the smell of damp or musty basements.
Damp shoes typically have a musty, stale odor due to the presence of moisture that promotes the growth of mold and bacteria. This smell can be described as earthy or sour, often reminiscent of wet fabric or mildew. The unpleasant scent can become stronger if the shoes are not properly dried or ventilated. Overall, damp shoes contribute to a lingering, uncomfortable smell that can be quite off-putting.