LIGO Open Science Center has published a signal processing tutorial about THE EVENT, the first observation of the gravitational waves on September 14, 2015, 09:50:45 GMT.
The scientists removed noises, apparent large oscillations and high frequency noise indiscernible on the screen, from the raw data by using Fourier transformation and notch filter. It is a little shocking that most sound we hear comes from the harmonics of fibers holding up the mirrors of the interferometers.
The sounds in the graphics are made in stereo. The right channel is from the LIGO Hanford Observatory, the left Livingston Observatory. Hanford observed the event 6.9 millseconds after Livingston did. So the sound runs from left to right theoretically
I never imagined how faint signals are that they are tackling with.
ref: Observation of Gravitational Waves from a Binary Black Hole Merger