The chimes of Big Ben.

Big Ben is the largest of the 5 bells in the Clock Tower, Houses of Parliament. The 4 quarter bells play the westminster chime roughly 23 seconds before the hour. After that, there is a 7 second pause, then Big ben strikes to mark the hour. Depending that the time is, Big Ben will strike accordingly, so if it's 1pm, Big Ben will strike once. Also, the 4 quarter bells strike every 15 minutes.