Space is a big ol' place, and so it's pretty easy to hide things inside of it. Frontier, it seems, has used that to their advantage. They've hidden a selection of strange, alien-like artefacts in Elite: Dangerous, and the community is just now getting around to analysing them.
Spurred on by the tease of non-celestial entities to discover, the community banded together in order to hunt down the mysterious artefacts. They even received some prompting from Elite: Dangerous's producer Michael Brookes, who suggested players "listened" to the cargo item. Yesterday, finally, the item was found.
Now the real work has begun. The community is in full ARG mode, attempting to analyse and decode the item's transmission. Here's a sped up version of the sound the artefact emits. It's fully possible speeding it up is not in any way helpful, but it does at least sound pretty creepy.
What does any of this mean? The suspicion is that it relates to the Thargoids—the insectoid aliens of past Elite games. An Elite: Dangerous dev diary from February 2013 specifically references the Thargoids, with Michael Brookes saying, "You will start to learn some of the secrets of the Thargoids, and they'll certainly be present in the game when it starts, but they'll very much be a background encounter." This latest discovery would certainly fit that bill.