In what was a bit of an unexpected find, Rosetta's Plasma Consortium picked up variations in the magnetic field around the comet, Comet 67P/Churyumov-Gerasimenko. The variations had audible frequencies, and the frequencies ranged from 40-50 millihertz. That frequency range is inaudible to the human ear, so some scientists at the ESA brought the frequencies into the hearing range of the human ear by altering the transmissions. The result was an extremely odd clicking sound. You can listen for yourself at the link:

This was an entirely unexpected development and is likely just the first snowball down the hill as we learn more about comets. Many scientists, upon learning of the sounds coming from the comet, are very excited to learn more about what process is causing these sounds to occur. According to IFLScience via CNET:

For now, the team's best guess is that neutral material that is shedding off of the comet is becoming ionized, or charged, by the solar wind. As of right now, the mechanism that would accomplish that task is not known.

Of note is that the sounds we hear coming from the comet are eerily similar to the Predator. Now, I'm not saying that a hyper-intelligent race of apex predators is using comets to scout for potential prey on nearby planets or anything, but I'm also not saying that this isn't the case. Either way, we definitely need to find out what this sound is.