From a Washington Post story by Carolyn Y. Johnson headlined “Scientists sent balloons into the stratosphere—and found a mystery”:

At first listen, Earth’s stratosphere seems calm and quiet. But when researchers launched solar-powered balloons up 70,000 feet, they detected a hidden acoustic world — including mysterious noises without a known origin.

The noises are “infrasound,” inaudible to the human ear — just as light in the infrared spectrum is invisible to the human eye. When recorded with specialized instruments and sped up a few thousand times, they sound like muffled, staticky whispers.