Flying object seen overhead travelling faster than the speed of sound

Outside a nightclub (can't remember the name), I and two companions heard a noise like thunder, only louder; a sonic boom, I believe.  I looked up, and passing overhead was a triangular object, apex in the direction of travel.  It measured approximately fifty metres end to end, and thirty metres wide.  
I wouldn't have though anything of this, apart for two reasons:  we weren't under a flight path there was no other noise; a silent propulsion engine. 
It circled directly above the city centre, then left at high speed heading north.