Last night around 22:00, my girlfriend and I were outside sitting and enjoying the night and we heard a helicopter flying low heading from the SW to NNE. It was flying low and had it's search light on. Shortly after the chopper passed, we heard another aircraft but it sounded different. The sound was almost like a heavy vibration, kind of similar to a train rumble in the far distance but not really. We then saw a triangle craft in the sky with a light at each point being escorted by another normal aircraft with flashing lights. They were heading NNE from S, very similar to the chopper that flew over before. Both the triangle craft and the aircraft escorting it were moving fairly slow and could be seen clearly for 6-7 minutes or so as they moved across the sky. The triangle craft looked as it was slowly rotating as it was flying, and the strange sound in the sky was coming from it.