@Nicole2592 I wouldn't offer anything. At this point you don't even know if you have rats in the attic or what. Squirrels or cats running around on the outside of the roof might sound as if it's coming from the attic, when it isn't. People who aren't accustomed to sounds like that might be freaked out about it, but there's nothing in that that warrants a discount.
If the inspectors do find evidence of mice or rats there, then I would offer to refund the guests unstayed nights if they choose to leave, but not otherwise.