The dark side of the moon has always held a mystical fascination for stargazers – so when the crew of one of the first missions to orbit it heard strange music in their headsets, they were so surprised they weren't sure whether or not to report it. The sounds were heard by the three astronauts on the Apollo 10 launch in 1969 while they were out of the range of terrestrial radio broadcasts.

Two months before Apollo 11 landed two men on the moon for the first time, the crew of Apollo 10 – Commander Thomas P. Stafford, Command Module Pilot John W. Young, Lunar Module Pilot Eugene A. Cernan – entered lunar orbit as part of a "dress rehearsal" for the Apollo 11 moon landings. This included spending an hour crossing the far side of the moon, which blocked all contact with Earth.

While they were out of radio contact with Nasa's mission control, they heard sounds described as "whistling" in declassified mission transcripts. Recordings of what happened were classified for 40 years, but they have recently been recovered, and the music is to be revealed on the Science Channel's series, Nasa's Unexplained Files.

The sounds lasted for the full hour of the crew crossed over the dark side. Mission transcripts reveal the three men talking about the noises and discussing what to do, but do not identify who was speaking.

One of the crew says: "You hear that? That whistling sound? Whooooooooo!"

"It sounds like, you know, outer space-type music," another replies.

Later, one says: "I tell you, that music is really weird."

"No one will believe us," replies a colleague.

On the show, Apollo 15 astronaut Al Worden says: "The Apollo 10 crew was very used to the kind of noise that they should be hearing. Logic tells me that if there was something recorded on there, then there was something there."

Possible explanations including a magnetic field, or atmosphere interfering with the radio are discussed on the show, but according to experts, the moon has no magnetic field and not enough atmosphere to cause interference.

Nasa has recorded "sounds" in space before, which aren't waves travelling in air, but electromagnetic waves that pulse through the vacuum of space. Several Nasa probes have recorded these waves and when they are translated in a form audible to the human ear, they can become quite unworldly.

"We'd had a lot of incidents where guys who flew in space saw and heard things that they didn't recognize, and you wonder about all of that," Worden told the Huffington Post. "I have a very open mind about what could've happened. It's somebody's hearsay evidence – it's only a visual or audio event, which is hard to pin down. Recollection is one thing, but actual proof is something entirely different."