I was walking through this one yard yesterday, and I'm walking passed some oil tanker cars. All of a sudden I hear, *ping, ping, ping!* so I stopped(scared the shit outta me), because I thought someone else was in the yard. It almost sounded like somebody throwing little pebbles into the tanker car. So I walked around to the front of the oil tanker, and listened again, *ping, ping ping!* Does anybody know what this is? Because it happened to every oil tanker car in the yard.