Mysterious Sounds Recorded During Near Space Balloon Flight

An anonymous reader writes: LiveScience reports on strange sounds recorded by microphones on board a high altitude balloon. The sounds were captured at altitudes of up to 36 kilometers, higher than any such experiment to date. Most of the noises are in Infrasound frequencies — below 20 Hz. Researchers aren't sure what caused the noises, but they have a few theories: "a wind farm under the balloon's flight path, crashing ocean waves, wind turbulence, gravity waves, clear air turbulence, and vibrations caused by the balloon cable."