Boom! Sounded just like one of the electrical transformers that blew out a couple of weeks ago down the street – which ended up being 4 big booms. I was already waking up, but that finished the job.
The boom this morning was then followed by the grumbly-ness of long, low thunder – and then by more thunder, plus lightning, followed by a wonderful but short rain.
Quite the way to wake up on the last day of August, 2013.