Knocking in Lawrence County, South Dakota

I travel, every year on vacation, to South Dakota to do some camping in the Black Hills and have for the past three years camped in the same location. I belong to a club of outdoor enthusiasts, we travel to certain locations to scratch a living from the earth for a few days and once you have the fever there's no such thing as a nonworking vacation. This last trip (June 2008) I had an experience I couldn't explain: I was asleep in my tent; I was the only person left in camp everyone else had packed up and left. I wasn't too concerned as I have camped by myself in this area several times and could only recall one other time when I felt like something was nearby perhaps a cougar or other predator type animal as the crickets stopped chirping and the whole area became extremely quiet. This particular night I was sleeping soundly and was then suddenly awake and aware that something was up, everthing was silent. I didn't know what had woken me at first but I reached over and checked my watch seeing that it was 3:11am I thought to myself I need to get back to sleep, then I heard it; someone or something was moving around in the stream below my camp. Not directly below, but more to the north and west of my camp following the curvature of the stream. Picture a curving "L" shape. Now rotate the "L" 1/4 turn to the right, apply that to moving stream water, my camp sits above the water 20 to 30 feet in the inside turn of the "L". I heard splashing in the stream and then, SMACK....SMACK. There was a pause of a few seconds then SMACK....SMACK....SMACK. It was two rocks being hit together in a pattern. Ok, creepy and not what I was expecting, I reached over and put a shell in the chamber of my weapon and checked the safety. I'll give it a few minutes and if nothing else happens I'll go back to sleep. Nothing else did happen except when the sun came up I packed up and left. The area around the stream is covered in rocks of every size and shape from small pebbles to boulders as big as cars and trucks. I didn't think to look for tracks. I was still the only one there and I didn't know what to think, except that if someone thought it was fun to show up at a secluded camp at that time of night to smack rocks together there must be something wrong with them and it's time to vacate the area. A few months later I saw a program on primates and how they will knock rocks and sticks together to communicate I put two and two together and realized I may have had an encounter.