Vampire Alert? ... :/

I know that to some (like me), believing in the existence of vampires is obviously ridiculous, but some strange things have been happening lately. Since about three months ago, I've been hearing really odd noises at night all around the outside of my house. At least a couple times each week I hear something running around outside and jumping lightly on the roof. I hear really weird scratching right outside my bedroom widow (which directly opens out onto the roof). I hear low screams and growls outside my bedroom window as well. Once in a while, my dog catches on and barks incessantly at my back porch, where some of the noises emanate from. I also hear scratching and quickened "footsteps" coming somewhere in or around my chimney. I've checked multiple times for raccoons and possums, and have managed to find some, but they've been taken care of, so they're not around any more. It's probably my imagination, because although I love vampire mythology, I hardly consider myself a "believer." Still...this has been going on for months and keeps me awake all hours of the night, and so far, I haven't found any other animals out there. What do you suppose is going on? No horror stories, please :) Just appropriate answers.