Whistling Acacia. Wilfred gave us a very interesting botany lesson. The brown lumps are called galls and are formed by the plant when it is damaged by the ants (look closely). The ants make holes in the galls and live there and come out to attack any herbivore that tries to eat the plant. A mutually beneficial relationship. When the wind blows through the tiny holes the ants make in the galls it makes a whistling noise - hence the name.