As a kid listening to some reletives who were farmers I heard them as they milked their cows. When a cow was a bit fidgity they would murmer to her "sook, sook, sook," and as she settled down they would change it to "sahh, sahh". And when they called the cows in from the field they would use the "soook!, soook!, soook!", a bit more drawn out and with a slight rising of pitch at the end. This was in southeastern Missouri in the 1940's.