First, you get the alarming news of an incoming airstrike. Or you don't.
Then you hear what can only be described as thundering sounds overhead. This is usually when you start to hear your own people firing gunshots into the sky, warning anyone in the vicinity of the airstrike. There is no need to point a firearm at your own people when the enemy is overhead. At night, these pretty little red bullets shine bright and illuminate the sky, shooting at an enemy they cannot reach.
Then comes the unmistakable thundering overhead, followed by a boom.