It could also be bats. I have bats in part of my attic crawl space, and I like them so well I've been negligent about evicting them. But both squirrels and bats will make themselves at home. (Although the bats should be settling down for hibernation.) See if you can investigate what their leavings and droppings actually look llike. -And put the cats in the attic overnight for a while, lol.