Alpacas make a variety of sounds such
as clicks and snorts but are best known and loved for their humming.
They make this humming sound in a variety of situations such as when
separated from their herd. When one female gives birth, the other
pregnant mothers may hum to the delivering mother and new cria, and
when grazing, they hum in different tones.
source: http://www.icstars.com/Ushop/Wara/alpacafacts.html