The next time you see some sludge metal band playing in drop-A or some other absurdly low tuning, remember the galaxy cluster Abell 426. At its center is a supermassive black hole, spraying jets of material through the hot gas that surrounds it. This generates bubbles in the gas which expand and set up acoustic waves. We know the speed of sound in the gas and we can measure the wavelength, so we can figure out the note. It turns out the black hole is playing B flat, 57 octaves below middle C. Slipknot are pikers in comparison.
For your GPOTD, I give you the Guitar Nebula. It's caused by the fastest known moving star, a neutron star moving 3.6 million mph plowing through interstellar gas- the shock wave forms the body with a nicely glowing headstock from the recent passage
Yeah, I know it's off topic, but I got the latest issue of Sky&Telescope that mentioned both and I enjoyed the collision of my two main hobbies.