1.   Checking for wet insulation in a wall or ceiling or floor is not easy, especially if it is not accessible.

2.   Plywood paneling or vertical solid boards can be dried out, but must be removed to expose wet insulation that must be removed.

3.   Rainy weather increases the emission of the odor from the already wet insulation.

4.   So, I suggest it is wet insulation in the walls, from a leak that never showed itself.

5.   The wet insulation assures filling cavities completely with no air pockets or other defects.

6.   Then remove wet insulation and get rid of it.