The answer to 'why is my mouse scratching sounds' is that mice often make scratching sounds as they move around and build nests inside walls, floors, or other hidden areas of your home. This scratching is a common sign of a mouse infestation, as mice are very active and need to constantly gnaw on surfaces to keep their teeth filed down.
The scratching sounds may be accompanied by other signs of mice, such as droppings, nibbled food packages, or small holes or entry points. Mice are resourceful pests that can fit through openings as small as a dime, so even a single mouse can quickly become a bigger problem if not addressed promptly.