OK, this has happened twice now, and scared the crap out of me.
I live on the first floor in my block of flats, immediately below my floor are the garages.
The garage under my flat has a car in it that only arrive (or started playing up) this week, I've never noticed it before. But it's really noisy, and every time it starts up it lets rip with a massive backfire that sounds like a gun shot.
It's got me a beauty twice now..