I had (grey) squirrels in the attic of my (upstairs) flat at one time. Besides chewing up everything we'd stored up there, they used to leap around on the plasterboard over our heads - and it sounded as if they were wearing size ten boots. I chased them out with a broom and blocked up the hole they were getting in through - it was great being able to sleep at night again. Thank god they never worked out how to open the freezer that lived up there !