Do you hear what I hear?

Gabriel will often run to find me and ask me if I heard a sound that he heard- "You hear train?" "You hear dump/ garbage truck/ car" "You hear a boom boom?"  The boom boom is probably the most common sound he mentions to me.  Usually he is delighted by the sounds he hears, but occasionally we hear a really loud sound from outside or the people upstairs, which scares Gabe, and he will scream or cry, and then ask me if I heard the "boom boom".  After reassuring him that I heard it, and talking about what might have made the sound, his curiosity is appeased and he goes back to what he was doing before.