Just a few months before Neil Armstrong and Buzz Aldrin made their historic moon landing, three NASA astronauts circled the moon as part of the Apollo 10 mission. This was a dress rehearsal of sorts for the actual moon landing mission, and as far as most people knew it went fairly smoothly. Yet the internet has recently been abuzz about the audio recorded during the mission, which captures the three startled crew members taking in the eerie sounds emanating from their radios.

One of Apollo 10’s main missions was to test the technology that allowed the spacecraft’s lunar lander module to detach and re-attach to the command module. But while the modules were separated for several hours, the crew members began hearing strange sounds, Bec Crew reports for ScienceAlert. The sounds were captured on an audio recording, below. Listen closely, the sound is faint compared to the background of the craft, but starts around minute 2:48 of the recording.

"You hear that? That whistling sound?" Apollo 10 astronaut Eugene Cernan asked his crewmates, according to a transcript of the mission. "Boy, that sure is weird music."

It’s not surprising that Cernan and his fellow crewmembers Thomas Stafford and John Young were freaked out when they suddenly heard eerie noises coming from their instruments, James Griffiths reports for CNN.

At the time, the Apollo 10 spacecraft was on the far side of the moon, out of contact with Mission Control and the farthest that any human has ever been from Earth. And the odd, high-pitched whine sounds like a stereotypical alien sound effect from a 1950s science fiction flick.

"You know that was funny,” Cernan said in the transcript. “That's just like something from outer space, really. Who's going to believe it?"

"Nobody,” Young answered. “Shall we tell them about it?"

"I don't know,” Cernan replied. “We ought to think about it some."

For decades, the freaky moment and audio clips went unnoticed until recently, when it made its way into a Science Channel program called NASA’s Unexplained Files, which dramatizes stories and small details from NASA mission files. While the transcripts and audio have been publicly available at the National Archives since the early 1970s, NASA only recently scanned and digitized the materials to publish on the internet, according to a recent statement.

Hearing weird, unexplained noises in orbit around the moon seems like something that would have been quite newsworthy at the time. But as Sean O’Kane writes for The Verge, it makes sense that the three astronauts would have downplayed the phenomenon. One of NASA’s highest priorities is keeping its astronauts safe, and this includes their mental health. At the time, astronauts and test pilots typically took a “lie to fly” policy towards any crack in their resolve, as any hint of psychological trouble could scrap a mission and ground an astronaut forever.

Decades later, it’s still unclear what caused the strange sounds. One possible explanation is that charged particles interfered with the radio communications between the separated modules, as scientists observed when the Cassini spacecraft passed by Saturn. However, unlike the ringed planet, the moon doesn’t have an atmosphere, ruling it out as a source for these particles. Of course, as Cernan says in a recent statement, it could have been something as simple as radio interference.

“I don’t remember that incident exciting me enough to take it seriously,” Cernan says in a statement. “Had we thought it was something other than that we would have briefed everyone after the flight. We never gave it another thought.”