Walnut husks = squirrel or chipmunk. Mice and rats don't eat them. Probably a squirrel as chipmunks will choose a ground den 99% of the time, if available. Squirrels build nests above ground.
I know some folks find a small hole and surmise it is a small rodent. Truth is, a squirrel can fit through a very small opening.
The best solution is to seal all openings with a material that can't/won't be gnawed through. Brass scrub pads and aluminum sheet fit that description. Spray foam, dryer sheets, and insulation products make very comfortable nesting material and rodent dentifrice.