NASA has released recordings of weird sounds Apollo 10 astronauts heard while flying around the far side of the moon, and the crew responding to the strange phenomenon.
"That music even sounds outer-spacey, doesn't it?" said astronaut Eugene Cernan to John Young, upon hearing the curious noise. "You hear that? That whistling sound? Whoooooo."
According to a NASA technician on the "NASA's Unexplained Files" TV show, the "radios in the two spacecraft [the lunar module and the command module] were interfering with each other."
A few months later, Apollo 11 pilot Michael Collins heard the same sound.
"Had I not been warned about it, it would have scared the hell out of me," Collins wrote in his book Carrying the Fire: An Astronaut's Journeys.
"Fortunately, the radio technicians (rather than the UFO fans) had a ready explanation for it: it was interference between the LM's and Command Module's VHF radios."