In May 1969, while passing over the far side of the Moon, the crew of Apollo 10 heard something strange. Cut off from contact with Houston back on Earth, the three of them were alone, listening to what they described as “outer-space type music.”

When communication channels reopened, the three astronauts failed to mention it to Mission Control, fearing that they would be concerned about their psychological state. However, although transcripts of the conversations the Apollo astronauts had were publicy released in the mid-1970s, this particular recording has only now come to light. As reported by CNN, the eerie, low-frequency whistling noise can also be heard on the recording.

“You hear that? That whistling sound? Whoooooo,” says astronaut Eugene Cernan on the recording, featured in a new Science Channel series called NASA's Unexplained Files (video below). “Well that sure is weird music.” Over the course of an hour, the crew continually comment on the surreal tune emerging from their headsets.

Perhaps unfortunately, this noise is unlikely to be Apollo 10 picking up on some alien orchestra. The two sections of the spacecraft – the Lunar Module and the Command Module – both had radios, and this whistling was likely to be the two interfering with each other. The noises began when they were both turned on in close proximity.

Michael Collins, pilot of the Apollo 11 lunar module, also heard similarly strange noises during his orbit of the Moon in July 1969, but he didn’t think too much of it. “Had I not been warned about it, it would have scared the hell out of me,” he wrote in his book Carrying the Fire, as reported by CNN. 

“Fortunately, the radio technicians (rather than the UFO fans) had a ready explanation for it: It was interference between the Lunar Module's and Command Module's VHF radios.”