Alicia Hotovec-Ellis is a doctoral student studying volcanic seismology at the University of Washington. She recently published a study in the journal Nature Geoscience along with some other folks that examines the sound emitted from Alaska's Redoubt volcano right before its eruption back in 2009.
Most of the time, human hearing isn't sharp enough to detect the sounds made by volcanoes. At least, not until they actually erupt. But, the 2009 eruption of Redoubt produced a pre-eruption sound at 30 hertz, which is a "very soft and very low hum, right at the limit of your perception," Hotovec-Ellis says.
That sound, often described as a "scream," happens because of harmonic tremors. These tremors are akin to a lot of tiny earthquakes and as the volcano gets closer to blowing, the tremors happen with a greater frequency. Eventually, the volcano goes completely silent and then explodes soon after.
Hotovec-Ellis adds, "because there’s less time between each earthquake, there’s not enough time to build up enough pressure for a bigger one. After the frequency glides up to a ridiculously high frequency, it pauses and then it explodes." So from the sound of things, it's not the screaming volcanoes we should be worried about, it's the ones that are silent instead.