NASA has declassified the unexplained audio files of 1969 Apollo 10 space mission during which astronauts had heard sounds from the 'dark side of the moon'. 

Here's the transcript of the conversation between astronauts Eugene Cernan and John Young when their space craft flew around the far side of the moon.

Source: NASA

The astronauts heard a whistling sound which stopped after a certain point of time. The files were 'confidential' files in 1969. Later in 1973 these files were made publicly available, NASA mentioned on it's website. 

Since there was no internet during the Apollo era, they recently provided digital files of some of the earlier missions. 

"I don’t remember that incident exciting me enough to take it seriously. It was probably just radio interference. Had we thought it was something other than that we would have briefed everyone after the flight. We never gave it another thought,"  said Apollo 10 Lunar Module Pilot Gene Cernan.

To hear the audio file click here

So do you think it's just interference or something more? 

Here's a serial that analysed the trip to the Moon: