After the sniper lights a cigarette, effectively giving away his location to his enemy, he realizes that the shot taken at him came from the roof of the building across the street from him. He cannot see his enemy because that enemy has hidden himself from view so that the sniper cannot shoot at him.
Just then an armoured car came across the bridge and advanced slowly up the street. It stopped on the opposite side of the street, fifty yards ahead. The sniper could hear the dull panting of the motor. His heart beat faster. It was an enemy car. He wanted to fire, but he knew it was useless. His bullets would never pierce the steel that covered the gray monster.
First, the speaker uses figurative language to describe the "dull panting" sound made by the motor of the armored car. Humans can pant, so this could be considered personification: the attribution of human qualities to something which is not human. Further, the speaker uses a metaphor to compare the armored car to a "gray monster," something hulking and massive and powerful. Notice that the narrator also calls it an "enemy car"; this could be considered an example of personification as well. It is not the car itself that is the sniper's enemy, but, rather, the enemy individuals with which it is associated. However, by calling it an "enemy car," the speaker seems to give it a life and motivation, a life and motivation shared with the enemies within the car.