Two birds were at least 1/2 mile away around Gundy Lake, Charlotte county N.B. They were not visible but could be heard (the sound was very odd). As they approached it was evident what the sound was they were making. One of the birds circled low. The other bird flew straight up to about 150' above and dove straight down to where the lower bird was flying. The noise I was hearing all along (at least 10 times before I knew it was birds) was the sound of the diving bird pulling out of the dive (I think). The air vibrations of this were quite loud, loud enough to hear at least a 1/2 mile away. Never witnessed it again and no one I know has ever heard of such a thing. Can some one tell me what kind of bird this was??