Evidently it is Not A Good Thing to hear this whistle. The whistle originates from The Lord and has some kind of judgement following it in each event.
- Therefore the LORD's anger burns against his people; his hand is raised and he strikes them down. The mountains shake, and the dead bodies are like refuse in the streets. Yet for all this, his anger is not turned away, his hand is still upraised. He lifts up a banner for the distant nations, he whistles for those at the ends of the earth. Here they come, swiftly and speedily! Not one of them grows tired or stumbles, not one slumbers or sleeps; not a belt is loosened at the waist, not a sandal thong is broken. Their arrows are sharp, all their bows are strung; their horses' hoofs seem like flint, their chariot wheels like a whirlwind. Their roar is like that of the lion, they roar like young lions; they growl as they seize their prey and carry it off with no one to rescue. In that day they will roar over it like the roaring of the sea. And if one looks at the land, he will see darkness and distress; even the light will be darkened by the clouds. - Isaiah 5:25-30
The verses before the above passage, especially verse 13, speaks about this judgement being an exile of God's people because "they have no regard for the deeds of the LORD, no respect for the work of his hands."
The next verse deals with the invasion of the king of Assyria on the land of Israel.
- The LORD will bring on you and on your people and on the house of your father a time unlike any since Ephraim broke away from Judah--he will bring the king of Assyria." In that day the LORD will whistle for flies from the distant streams of Egypt and for bees from the land of Assyria. They will all come and settle in the steep ravines and in the crevices in the rocks, on all the thorn bushes and at all the water holes. - Isaiah 7:17-19