Possible meanings of "the loud noise" are 1) it refers to Yahweh's voice. AT: "The peoples flee at the sound of your loud voice" or 2) it refers the loud sounds of Yahweh's army. AT: "The people flee at the sound of your army" (See: [[rc://en/ta/man/translate/figs-explicit]])
This compares how quick and eager Yahweh's people are when they gather the spoils from their enemies to the eagerness of locusts when they gather food. This can be stated in active form. AT: "Your people gather spoils from your enemies with the same fierceness as the locusts have who devour green plants" (See: [[rc://en/ta/man/translate/figs-simile]] and [[rc://en/ta/man/translate/figs-activepassive]])