I didn't read the other replies (sorry, not much time), but we had this happen.
Usually the squirrel left the attic during the day, so MMM went up there when he was pretty sure the squirrel was gone to check for nests and babies, etc. Then, he climbed up a ladder to the roof and found the spot where the squirrel had entered and he plugged it up with some metal and I think some foam insulation.
Actually, he discovered the hole first, got the supplies, checked the attic, and then plugged up the hole. :)
Good luck! Hearing that squirrel scampering about in the attic every night always freaked me out!