Europe's Rosetta spacecraft has discovered something surprising about the comet it's orbiting: it makes an eerie noise.

The European Space Agency robot's instruments have detected a low-frequency, irregular sound emitted by oscillations in the comet's magnetic field. Researchers have boosted the sound to within human hearing range and are calling it a "song."

The comet's song can be heard in a clip posted online. The song changes rhythm and pace, going from what sounds like a gurgle, to a series of clicks, to the sound of wind blowing and back to more clicks that constantly change pitch.

The ESA said it first detected the sounds in August, when the Rosetta orbiter drew within 100 kilometres of the comet, dubbed Comet 67P/Churyumov-Gerasimenko. It picked up the sound during a magnetometer experiment.

Scientists say they don't know the "precise physical mechanism" behind the sound, but it appears to be caused by particles released by the comet becoming charged as they move out into space.

The comet's song is being sung at 40 to 50 millihertz, well below the threshold of human hearing.

On Wednesday, the Rosetta orbiter's Philae lander touched down on the surface of the comet, where it will gather data about the makeup of the giant space rock over the coming days.

The Rosetta orbiter is expected to stay with the comet for years, gathering data that scientists hope will provide clues to the origins of life on Earth.

The comet is hurtling through space at about 66,000 kilometres per hour, about 500 million kilometres from Earth.