C. F. Keil and Franz Delitzsch on Isaiah 5:29

Isa 5:29 “Roaring issues from it as from the lioness: it roars like lions, and utters a low murmur; seizes the prey, carries it off, and no one rescues.” The futures, with the preceding לו שׁאגה which is equivalent to a future, hold each feature in the description fast, as if for prolonged contemplation. The lion roars when eager for prey; and such is now the war-cry of the bloodthirsty enemy, which the prophet compares to the roaring of a lion or of young lions (Cephirim) in the fulness of their strength. (The lion is described by its poetic name, לביא; this does not exactly apply to the lioness, which would rather be designated by the term לביּה.) The roar is succeeded by a low growl (nâham, fremere), when a lion is preparing to fall upon its prey.In Arabic, en - nehem is used to signify greediness (see Ali’s Proverbs, No. 16).

And so the prophet hears a low and ominous murmur in the army, which is now ready for battle. But he also sees immediately afterwards how the enemy seizes its booty and carries it irrecoverably away: literally, “how he causes it to escape,” i.e., not “lets it slip in cruel sport,” as Luzzatto interprets it, but carries it to a place of safety (Mic 6:14). The prey referred to is Judah. It also adds to the gloomy and mysterious character of the prophecy, that the prophet never mentions Judah. In the following v. also (Isa 5:30) the object is still suppressed, as if the prophet could not let it pass his lips.