Shepherd's whistle
They are also produced commercially. Simple whistles may cost less than 1 USD in bulk; expensive ones may be quite ornate. They are commonly made of plastic, aluminum, stainless steel, silver, brass, titanium, Corian, jade, buffalo horn and other materials.
Whistle
They also occur as accidental byproducts of fluid flow such as supersonic jets, cavity resonances, whistling telephone wires, and idling circular saws.