The technical term for this sort of thing is hypnagogia (http://en.wikipedia.org/wiki/Hypnagogia); if you want to dig deeper, that's where to look. It's sort the sensory parallel to when you're just starting to fall asleep and suddenly your leg kicks and wakes you up. (http://en.wikipedia.org/wiki/Hypnic_jerk)

I've been on the edge of sleep and thought the gurgling noises from my radiator were mumbled speech outside my window.