Could It Be Mice?
It’s not that uncommon for animals, especially small rodents like rats and squirrels, to make little homes inside of the walls of your house. When the weather gets cold, they look for places to make burrows for the winter.
The thing is, these noises had been going on for a while. It was September and it wasn’t cold yet. The noises weren’t going away, they were only getting louder and more intense.