Charles Ellicott Commentary 2 Samuel 5:24

"And it shall be, when thou hearest the sound of marching in the tops of the mulberry-trees, that then thou shalt bestir thyself; for then is Jehovah gone out before thee to smite the host of the Philistines." — 2 Samuel 5:24 (ASV)

The sound of a going. —After David has gone to the rear of his enemies, he is to wait by “the mulberry trees,” or, as now generally understood, baca-shrubs, a plant resembling the balsam. Here a Divine signal was to be given to him in “the sound of a going,” or, rather, of a march. The word is used of the march of the hosts of the Lord in Judges 5:4; Psalms 68:7. Then David was to “bestir himself,” literally, be sharp; he was to act quickly and vigorously.