Humid summer days and rainy days adding moisture will produce undesirable odors from a dirty chimney. Having your chimney cleaned will dispose of any decomposing materials and will also clean the creosote out of the chimney. If the chimney cleaning and inspection don’t get rid of the odor, the next course of action would be to seal the chimney from water sitting in the chamber and causing a musty smell. Adding a chimney cap to your chimney will keep water, debris, and animals out of your chimney. Below is a list of what your chimney might smell like with what is most likely causing that smell.
Animals entering your chimney and not being able to escape
Fallen leaves combining with damp soot
Water sitting in your smoke chamber
Most likely due to creosote buildup