The skull whistles remind me of an experience in Central Asia. I was in an expedition camped out in the middle of nowhere, a day's drive from the nearest permanent settlement and miles from the nearest nomads. It's the middle of the night and this terrifying scream like a woman being murdered wakes me up. It was right on the edge of sounding inhuman in the same way.
The sound kept happening as I made my way down the ravine in the direction of the sound and at the bottom... Camels. Just a normal herd of camels fording the river.