I just saw a program on NOVA about this, and it is extremely neat.
In some places around the world, sand sings. Marco Polo heard it in the Gobi, Darwin heard it in the Chilean desert, and it has been documented from the Middle East since the 400s. The peculiar geographical feature is called the Booming Dune.
The booming dune has puzzled travelers for centuries: it has been known for a long time the sound in fact does not come from the wind blowing over the sand dune--the sound comes from the sands themselves. The conditions required for a sand dune to boom is critical--there are, after all, only around 30 places in the world with booming dunes. Some observations suggest that it is intimately tied to the wind: the wind-polished particles of sands have the necessary spherical shape and smooth surface, and are dry and uniform in size.
Melany Hunt, a professor of Mechanical Engineering at California Institute of Technology, specializes in granular and liquid-solid flows. Her hypothesis, expanding on previous conjecture of Bagnold, is that the sound is produced by the sliding of the dry, fine-grained, wind-polished sand on the surface of the dunes. The sound then gets reflected and amplified in some sort of wave-guide phenomenon by the wetter sand deep in the heart of the dunes. Comparing against the guitar: the dry sand layer is the string, and the wet sand is the resonating chamber.
To test her hypothesis, Professor Hunt leads students to the Mojave or Death Valley, and slide down the sand dunes on their pants, creating a mini-sand-avalanche. And indeed, it verified parts of her theory: that the sliding sand does indeed produces a booming sound in the characteristic frequency ranges recorded for the Booming Dunes.
further readings: