Definition of "grunting"

Tired with the heat and my walk, I threw myself down on the grass in the shadow of the house, where I lay half-asleep enjoying a quiet rest, when I was startled by an unpleasant clamour, the jarring voice of a woman, who was trying by alternately scolding and using pet names to pacify a litter of grunting pigs on the farm.

1886, Peter Christen Asbjørnsen, translated by H.L. Brækstad, Folk and Fairy Tales, page 278