Same noise with my civic as well. I checked the handbrake if its losing its grip, disconnected the battery, emptied the patrol tank, even checked for a cat underneath but nothing. There is no continuaty, some times it sounds 2-3 times in an hour, sometimes nothing for days. Its quiet scary when someone pass by it in night and hear it.
The conclusion I came to is that its actually haunted :o But don't worry, the ghosts stay only for a couple of months, then they move to another location, besides they provide added security!