That is the sound of the gravitational waves hitting the LIGO detector. A chirp.
That is also the sound of the celebratory hurrah’s from the gravity community. We finally have experimental (observational) confirmation to a prediction made by Einstein’s theory of general relativity 100 years ago.
The quest to hear gravitational waves started about 50 years ago by Webber and it is only now that enough sensitivity is available in the detectors to be able to hear the ripples of spacetime as they pass through the earth.
The particular event in question turned the equivalent of 3 solar masses into gravitational waves in a few seconds. This is much brighter in power than the brightest supernova. Remember that when supernova collapse, the light emitted from them gets trapped in the shells of ejected mater and the rise of the signal and afterglow is extended to months. This was brighter in energy than all the output of all the stars in the visible universe combined! The event of Sep. 14 2015 recorded the merger of two black holes of intermediate masses (about 30 solar masses each) about 1.3 billion lightyears away.
The New York times has a nice movie and article to mark this momentous scientific breakthrough.
Congratulations to the LIGO team.
Usual caveat: Now we wait for confirmation, yadda yadda.