In May 1969, Apollo 10 staged a dress rehearsal for the first human landing on the lunar surface. The mission was a success, with the crew of three orbiting the Moon for over 10 hours, and separating and then rejoining a lunar landing module.
While they were up there, alone on the far side of the Moon, the astronauts reported hearing a "whistling sound" which one said "sounds like, you know, outer-space music". The sounds, which were recorded by cameras on board Apollo 10, were not released to the public for nearly 50 years.
According to transcripts, Apollo 10 astronauts Thomas Stafford, John Young, and Eugene Cernan discussed the spooky sound they heard:
Young: Did you hear that whistling sound, too?
Cernan: Yeah. Sounds like – you know, outer-space-type music.
Young: I wonder what it is.
NASA warned the crew of Apollo 11, including Michael Collins, who flew the command module alone while Neil Armstrong and Buzz Aldrin headed to the Moon's surface, to look out for the sounds.
“There is a strange noise in my headset now, an eerie woo-woo sound,” Collins wrote in his book Carrying the Fire: An Astronaut’s Journeys. “Had I not been warned about it, it would have scared the hell out of me."
The noise began after the lunar module (LM) separated from the main module and ended when it landed on the Moon. Fortunately, by the time Collins heard the noise – which happened to be when NASA was out of contact for an hour, as expected – it had already been explained.
"The radio technicians (rather than the UFO fans) had a ready explanation for it," Collins wrote. "It was interference between the LM’s and Command Module’s VHF radios.”
NASA confirmed this explanation when they released the video footage in 2016.