"Rattlers in the stomach" was an Earth idiom that referred to the state of being nervous, which in Humans may be accompanied by a "fluttery" or "rattling" sensation in the stomach. Unlike the similar expression "butterflies in the stomach," it did not have connotations of romantic excitement.
Rather than pretty, delicate butterflies, the expression may evoke the idea of a rattlesnake, sometimes referred to as a "rattler." The snake has an organic rattle at the end of its tail that it shakes as a warning before it strikes, and is a potent symbol of imminent danger.
Jack Crichton asked his son, John, if he had rattlers in his stomach before John's test flight aboard Farscape-1. John denied it, pointing out that he'd been up in the shuttle twice before. Jack replied, "Didn't matter how many times I went up. Every time: rattlers."[1]
Later, after John's encounter with the wormhole and various aliens, he recorded a message for his father admitting, "Dad, you know those rattlers in the stomach we talked about? Well, I got 'em now...."[1]