9:9 And they had breastplates like breastplates of iron; and the sound of their wings was like the sound of chariots, of many horses rushing to battle.
These locusts or demon creatures are well protected, they think. They cannot be killed, nor can they kill; they can only torment for 5 months.
Note the contrast between these demons "protection" and the believer's BREASTPLATE (1 Thessalonians 5:8) which is given to them by Christ who wears His BREASTPLATE of righteousness, Isaiah 59:17.
The demon creatures have movement and their movement is loud. It is not as though they sneak up on you. Those unsealed persons will hear them going and the SOUND alone should strike terror into their hearts. Joel sees them as leaping from the top of one mountian to another, Joel 2:5 (or can we interpret this as metro area to metro area?).