Leviticus 26 warns that survivors in enemy lands would flee at the sound of what?

Answer: A windblown leaf

God warned of such faintness of heart that "even the sound of a windblown leaf will put them to flight" (KJV: "a shaken leaf").

“As for those of you who survive, I will send a faintness into their hearts in the lands of their enemies, so that even the sound of a windblown leaf will put them to flight. And they will flee as one flees the sword, and fall when no one pursues them.”— Leviticus 26:36 (BSB)