By any chance have you noticed a lot of bees in your area lately? There's bees called carpenter bees that tunnel through wood to make their nest. As a kid, I thought they ate the wood, because if you walk close to a nest (about 5-6 feet) when it's quiet (like at 3 am) you can hear a munching, scritchy/scratchy kind of sound. I later read that they actually just chew it up to make tunnels, and spit out the sawdust or use it to help build their nest. They look a lot like bumblebees.
Could also be mice as suggested by others, termites, or even squirrels. Squirrels sometimes decide to nest in attics or eaves and other roof spaces of buildings, especially if there's a nice little highway for them to use like a power line near a vent or tree branch that touches the roof. I remember not so long ago there was a post by someone here about a mystery bird that managed to get get down their chimney and scare the crap out of them by making noise in the basement-another possible explanation.