The black hole resides in the Perseus cluster of galaxies located 250 million light years from Earth. In 2002, astronomers obtained a deep Chandra observation that shows ripples in the gas filling the cluster. These ripples are evidence for sound waves that have traveled hundreds of thousands of light years away from the cluster's central black hole. [...]
In musical terms, the pitch of the sound generated by the black hole translates into the note of B flat. But, a human would have no chance of hearing this cosmic performance because the note is 57 octaves lower than middle-C. For comparison, a typical piano contains only about seven octaves. At a frequency over a million billion times deeper than the limits of human hearing, this is the deepest note ever detected from an object in the Universe.
Update: danfuzz notes:
A little calculation reveals that the period of the "sound wave" is about 20 million years. Yay google calculator:
1 / ((middle c / (6th root of 2)) / (2^57))
= 19 593 253.1 years
"middle c / (6th root of 2)" is the b-flat below middle c.