The crew of NASA's Apollo 10 mission in May of 1969 set the groundwork for showing that Apollo 11's moon landing would be possible. They flew around the moon and practiced separating and descending the lunar module to better understand potential issues with landing.

But — in information that wasn't made public for decades — they heard some mysterious sounds in the process, an eerie whistling that they couldn't understand and weren't sure how to report.

As lunar module pilot Eugene Cernan asks John Young, the command module pilot:"That music even sounds outer-spacey, doesn't it? You hear that? That whistling sound?"

text from the transcript of their discussion was released in 2008. Thatof their discussion was

Now, NASA has declassified the actual audio from the mission, which made its way into an episode of the Science Channel show "NASA's Unexplained Files." It's tough to distinguish the show's sound effects from recorded audio, but if you skip to 2:16 here you can catch some of the whistling:

Here's part two of the promo for the show: