Bats!
We had bats in our attic. We think someone nearby did a bat removal last year and they found their way to our home. We had to wait until the end of September to remove the bats via exclusion with bat cones and then we sealed off the attic so they could not return. These are Indiana Little Brown Bats and are an endangered species. Bats are very important to the ecosystem and are awesome mosquito eaters. Plus they are really neat to watch. Luckily, the bats were only present a season so there was almost no cleanup but we still went to be tested for histoplasmosis just to be safe.