The lee face of a tall dune fails in shear. Sand slumps. Each grain in the moving layer collides with its neighbours about a hundred times a second, and the elastic waves from those collisions bounce off the free upper surface of the slide and synchronise. The dune's body is, roughly, a five-foot dry crust over damper sand beneath. The crust traps the resonant frequency. The slope amplifies it. A booming dune in the Namib will hold a single tone — near concert A, around 450 Hz — for minutes at a time, at up to 105 decibels.
The sound stops when the sliding stops. The avalanche is not the trigger of the note; the avalanche is the body of the note. Whatever you call the instrument, the played part of it is the sand mid-fall. When the slumping is done the dune stands silent again, mostly restored, very slightly shorter at the crest. Each note costs surface.
The wind built it. Round silica grains between about 0.1 and 0.5 millimetres, sorted by air, layered by humidity, the upper crust dried by the same wind that delivered the grain. No one designed this geometry. Selection without an aim. The dune does not know it is an instrument. It does not know what is about to happen when its lee face passes the angle of repose. It is a thing that, under one specific kind of failure, sings a specific note.
The note is the strange part. Pitch correlates with grain diameter, predictably and tightly. Larger grains, lower note. The relationship has been measured many times, by many groups, in different deserts. It holds. The mechanism by which a grain of a given diameter, sliding at a given shear rate, produces a tone of a given frequency — that is unsolved. Not "no one has bothered" unsolved. Actually unsolved. The resonator is understood. The trigger is understood. The self-synchronisation is understood. The mapping from a 0.3-millimetre grain to a 450-hertz tone is a paper with a gap where the derivation should be.
You can stand at the foot of a slope in the Namib and hear it. Microphones get the same waveform. The waveform repeats. The frequency is reproducible to within a few hertz across decades and continents. We can record the answer and we cannot yet write it down.