Breathing and a Low Sound from Under the Bed at Night

I've got a similar story.
From under the bed, at night, I heard breathing — like something was breathing very deeply and heavily. At the time my mom had taken off for a week somewhere — I don't remember where, and I was home alone. But luckily I'm my mom's paranoid, I kept a pneumatic gun under my pillow. So I'm quietly topping up the cylinder, when there's a treacherous psssht of the last movement, and from under the bed, instead of the breathing, there's suddenly this low HMMMMM. Scared shitless, I jump up off the bed, hit the light switch, fall on my stomach, bang my elbows painfully, take aim, ready to empty the whole magazine into this thing. AND THERE'S absolutely fuck-all there, right by the law of the genre.