Runner Hears Unusual Whistle Near Woodsford witnessed in Alpine County, California

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While walking cross-country, looking for possible home sites from the early 19th and late 18 hundreds, I was walking the base of a small ridge between two lakes Lower Blue and Twin Lake. On the ridge side it was mostly open forest on the Twin Lake side the brush got thicker. As I was walking I heard a loud two tone whistle [low to high pitch] that was not a bird or any animal I know. The sound came from ground level and behind me to my left. The whistle was loud and sounded in my opinion from a mammal. I turned and called out thinking a hunter [in a place he should not have been][no hunting zone] but no one answered, I whistled back but nothing called. Wondering what it was, I investigated the area 'till i could get no farther, the brush was too thick. I was standing within 10 feet from where the whistle came from, nothing in sight not in the trees on the ground not anywhere, no bird's where flying and no animals in sight either. It was a little spooky but at the time i was cross-count...