A thin, solid aluminum rod. When you hold it at a nodal point and rub your fingers across it, a transverse wave is produced that bounces back and forth inside the rod. The more you rub the rod, the larger the wave becomes. In a rather short period of time, you will be producing a very loud, obnoxious hum that will annoy even the most ardent heavy metal fan.
LL Demos & Labs: Singing Rods