An audio tape of a conversation between Apollo 10 astronauts about weird music heard on the far side of the moon has just been released by NASA.

The transcripts were released in 2008 but this is the first time that audio of the discussion and the sounds themselves have been released, according to a CNN report.

The Apollo 10 mission launched on May 18, 1969 and served as a “dress rehearsal” for the moon landing by Apollo 11 later that year. It conducted all activities short of actually landing, orbiting around the moon before heading back to Earth.

It was while on the far side of the moon during the orbit that the astronauts reported hearing strange sounds. They described it as a whistling sound, or “weird music.” They were concerned about reporting it to their superiors as it might call their mental stability into question.

But they weren’t crazy at all, and scientists think they have an explanation for hte sounds. In fact, it was NASA’s own doing: the radios in the lunar module and the command module were interfering with each other at the time.

In fact, the pilot of Apollo 11, Michael Collins, also heard the sounds when he was on the far side the moon. Collins did not join Buzz Aldrin and Neil Armstrong in heading to the lunar surface. He also reported an “eerie woo-woo sounds,” according to the report. He said it would have scared him had he not been warned about it. The “woo-woo” noises apparently stopped when the lunar module landed on the moon’s surface, ending the interference.