If they hold their wings flat over their abdomens, then they are likely cluster flies. These flies enter the walls and attics of homes in large numbers to survive the winter. On warmer days, they may crawl into the living spaces of the home. Once inside the walls, they are difficult, if not impossible, to eliminate. You need a professional to inspect the home and make recommendations for sealing cracks where flies could enter. Treatments can be applied that can greatly reduce the numbers of flies seen inside.