Screaming or howling in Roosevelt County, Montana

My half brother Shawn Flaten, my friend Monty Yellowbird ( a local Black belt karate instructor from South Dakota), his younger brother( forgot his name, and myself(Kevin Martin),all were hunting on the reservation (I am a tribal member and we could hunt year round). We had decided that we would go by a place I had picked (by the Stiffarm residence), which was down by the river. The river the Missouri about 30 miles or so from the FT Peck dam, had some good areas that a fairly thick in brush, open fields and Cottonwood trees. I was told by various tribal elders not to go down to the river at night, but since we had rifles I didn't really think about danger. We were on the old Indian highway heading west and then we drove about one fourth mile past the Stiffarm's residence and turned South down a road that dead ends after about another quarter mile. Montie's brother said to let him off at an irrigation canal that ran due east/west he was to walk all the way east until the open field on his left and the brush on his right ran into the trees. The trees that the irrigation canal ran into was a "deep" woods area that hardly anyone went into, mainly because it was owned by a non Indian and partly because it was a little spooky and quiet. Monty , Shawn and myself stopped the car at the turn around spot and got out. We then headed South and crossed a barbed wire fence and got on another irrigation canal ( parallel to the one Monty's brother was on). It was about 5:30 PM just about enough time to get a deer and get out before dark. We started walking east and passed a block of brush to the left and came upon an open field. Monty said that he was going across the fields to the east, right below were his brother was and start flushing in the brush that was all on the east border of the field. We bid him good luck, and continued on. I told Shawn to set up on the south east corner of the field next to the irrigation canal and wait to see if one of us flushed something. I continued on and went pass the block of brush Monty was in and turned North. The river was on my right and the brush was on my left, and I started walking up the lane. About this time, I knew that I had miscalculated the light and it was getting dark fast. The road up ahead was dark and it went into the "thick forest", right at the point were the irrigation canal that Monty's brother was on entered the woods. I was walking slow listening and then all of the sudden right in front of me about 100 yards ( sound guess) there was this horrible and very loud screaming. The sound was deep and had gurgling noises mixed in with a hooting like a chimpanzee. The sound was right on top of were I had mentally placed Monty's brother to be. I was told by our elders to stand your ground as a warrior if one of those ever came to you. I looked at the .22 in my hands and a broad yellow streak went up my back. I spun around and took off back to the canal. I ran around the corner and about 150 feet later ran into ( almost literally) Shawn. He said later he almost shot me, and we ran west on top of the canal. We stopped about right in the middle of the field on our right and spotted something running towards us from about were Monty went in the brush. Shawn said " should I shoot it ", then I said "yea". Shawn drew up his gun and was taking aim when he decided to wait a few more seconds. Its a good thing because Monty came running all out of breath, and said "do you guys hear that?". All the while it was continuing to scream, gurgle and hoot unlike anything I have ever heard. We ran all the way back to the car and piled in. We took off north and met Monty's brother already standing on the road. He was shook up pretty bad and we asked him what happened. The weird thing was that he never told us, then, nor later on. Monty though it was us for about 2 seconds and then decided based on sheer terror that no human could make those sound that long. It was definitely the closest and most chilling, as far as immediate danger goes. I never saw Monty or his brother after that, and even my brother and I don't talk about it much. I am spooky about the military, because the people that filmed them in this location before were professionals, and the Indians I talked to said that they even got daylight footage, yet nothing was ever in the news. They even had pictures of a group of them, yet nothing.... That is why I am spooky about it.