If you hear a tapping noise in your wall, it's likely caused by an animal. Mice and rats are the most common culprits, but other animals like squirrels and chipmunks can also make this type of noise. The tapping is usually caused by the animal trying to gnaw on something, either to create a new opening or widen an existing one. If you think you have an animal in your wall, the best course of action is to call a pest control professional who can safely remove the animal and seal up any openings to prevent future problems.