Why do I get auditory hallucinations of sirens?

Every now and then I'll be lying in bed falling asleep when the noise of police sirens start creeping up on me. First it'll be distant, like part of a dream, then it gets louder until it sounds like they're inside of my room or outside of my window. Usually I have a fan on, and I'm thinking that it may have something to do with my ears distorting that sound, but sometimes the noise persists even after I've turned the fan off. Also, I live in a heavily forested area, and the chances of cop cars driving like mad crazy around my house at night are slim to none. So, does anyone know what's going on?