What the heck is that noise?

So I'm laying in my bedroom and it's 2 in the morning, I'm on yahoo answers right now on my iPod when I should be asleep. Anyways, there's this noise upstairs that I can hear, it sounds like a vacuum cleaner. Let me remind you, it's 2 in the morning & everyones asleep. Is it all in my head or is it actually a vacuum cleaner. I'd go upstairs but I'm too tired. Plus there's a thunder storm outside. I'm really flustered & tired right now. I also live in wisconsin. So what's that noise?