It seems when Apollo 10 explored the far side of the moon, it uncovered strange whistling sounds which were classified by NASA for decades.
It's not that unusual to detect odd sounds in outer space, as these recordings of star oscillations show. What's much stranger is uncovering music from the far side of the Moon. Or learning it was recorded during the Apollo 10 mission, buried in NASA's classified archives for 40 years, and emerged to spark inevitable "but is it aliens" debates all over again.
In May 1969, Apollo 10's astronauts prepared for a lunar landing by flying across the Far Side of the Moon. During this time, NASA had no contact with the spacecraft, but recorded all conversations and stored the transcripts in its archives. The recording were declassified in 2008, and have since revealed whistling sounds over the radio which astronauts Tom Stafford, John Young, and Eugene Cernan discussed at length.
"It sounds like, you know, outer space-type music," an astronaut says in the recording. The sounds cut out after an hour, prompting a discussion about whether they should be mentioned to NASA.
While there's no record of Apollo 10 astronauts or NASA discussing these sounds, Apollo 11's Michael Collins' described the phenomenon in his 1974 book Carrying the Fire. But the general public only picked up on these sounds fairly recently, such as when they were covered by Science Channel's NASA's Unexplained Files last weekend. Despite what the episode implies, there's a perfectly reasonable explanation - VHF radio interference between the lunar and command modules.
Of course, that answer won't be good enough for everyone, including astronauts themselves. "The Apollo 10 crew is very used to the kind of noise that they should be hearing," Apollo 15 pilot Al Worden said. "Logic tells me that if there was something recorded on there, then there was something there."