The first time we hear this phrase is when the dinosaur crashes into the jungle. The noise it creates is like the roar of thunder during a tremendous storm. It scares Eckels enough that he has to hurry back to the time machine.
The second time the phrase appears is at the very end of the text. The time travelers come back from the past only to discover that the course of history has somehow changed. Travis takes up his rifle in order to right this wrong. Bradbury ends the story with the quick and ambiguous note, “A sound of thunder.” What really happens here? Evidently someone gets shot, and the acoustics cause the impact to resound throughout the office. We are left to wonder. Does Travis kill Eckels, or himself? A solid case could be made for either alternative. He could have killed Eckels for stepping on the butterfly. Alternatively, he could have killed himself in order to absolve himself of the responsibility for the accident leaving Eckels to live with the changes he has inflicted upon the world. What do you think?