When something noisy is coming towards you, the noise gets louder and louder, until the thing passes nearest you (it's loudest then) and then it gets less and less loud

Now, if that thing is going faster than sound, you won't hear it coming. The thing travels faster than its own noise. There is no noise build-up as it approaches. Then it passes you, and the noise level goes from 0 to loudest all at once. One second later, the thing is hundreds of meters away and way less noisy. So it should like a thunderclap. The sonic boom.