"X-Files"-like sounds recorded far above the Earth's surface are baffling NASA scientists who are yet to pinpoint their origin.

Daniel Bowman, a graduate geological science student at the University of North Carolina, recorded the mysterious noises in August, using infrared microphones positioned nearly 40 kilometres above earth.

The microphones were sent up in a NASA-funded balloon to hover above the US states of New Mexico and Arizona.

The sounds, picked up at frequencies below 20 hertz, the threshold for human sound detection, could only be heard by human ears after being sped up.

"It sounds kind of like The X-Files," Mr Bowman told Live Science yesterday.

Researchers do not know the source of the noises, with some suggesting they came from wind farms or cable vibrations, while others are rushing to conclude they come from an alien race.

Douglas Vakoch, the Director of Interstellar Message Composition with the Search for Extra Terrestrial Life project, has founded a site called Earth Speaks to gauge what people would like to say to aliens in the event of contact.

Mr Vakoch said while women tended to offer messages of friendship, men were more likely to ask about their civilisation and technological advances.

Overall the main theme was asking for help from aliens rather than seeking to impart the wisdom of Earth, with humans having a "cosmic inferiority complex".

"Humankind has a range of experiences and insights that cannot be imagined by any other civilization," Mr Vakoch said.