Apollo 10 has a unique place in the history of the moon landing program, in that it was the "dress rehearsal" for the ultimate moon landing on Apollo 11. As such, one of the mission requirements was to detach the lunar module from the command module, fly the lunar module very close to the moon's surface without landing, and then redock the two vehicles.
Sounds simple enough, but it turns out something weird occurred as the crew was carrying this out. While on the far side of the moon, with radio contact to Earth cut off, the crew started hearing strange sounds coming through their radios. Here's a transcript, uncovered in 2008, of astronauts Thomas P. Stafford (CDR), John W. Young (CMP) and Eugene A. Cernan (LMP) discussing what they're hearing.
Audio of the conversation, including the "outer-space-type music," was not widely available until now, but thanks to a new Science Channel series, NASA's Unexplained Files, we get to hear some of it. Check out the clip below, including commentary from Apollo 15 astronaut Al Worden.
So, is there an explanation? According to a NASA expert, the noise is caused by interference between the radios on the lunar and command modules, when both vehicles' radios are on and close to each other. This explanation is backed up by Apollo 11 Command Module Pilot Michael Collins, who notes in his memoir Carrying the Fire: An Astronaut's Journeys that he was even warned by radio technicians to expect some odd sounds.
"There is a strange noise in my headset now, an eerie woo-woo sound," Collins wrote.
"Had I not been warned about it, it would have scared the hell out of me (...) 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."
Collins went on to note that, when the Apollo 11 lunar module finally landed on the moon (something, remember, that Apollo 10's module never did), the noises stopped.
So, there you have it. It's a perfectly simple explanation ... unless, you know, you believe there's a Nazi Moon Base hiding back there.