OK, this is too cool. From Physics News Update (with thanks to Strata for the link):
For centuries, world travelers have known of sand dunes that issue loud sounds, sometimes of great tonal quality. In the 12th century Marco Polo heard singing sand in China and Charles Darwin described the clear sounds coming from a sand deposit up against a mountain in Chile. Now, a team of scientists has disproved the long held belief that the sound comes from vibrations of the dune as a whole and proven, through field studies and through controlled experiments in a lab, that the sounds come from the synchronized motions of the grains in avalanches of a certain size.