Almost surely a rodent. If you can locate the EXACT place where the scratching is happening and REALLY want it over with, and if the material is drywall, you can carefully cut a hole large enough to insert a well-baited trap (peanut butter is good) and tape the cutout section back into place until you catch the critter. Should not take even 24 hours. As someone said, be sure to attach a tether to the trap. When the deed is done and trap/rodent removed, you can glue a splint to the back of the cutout, glue the splint to the inside of the wall, and refinish the cutout spot.
