From The New York Times:
_It may not rank as one of the great scientific mysteries of all time, but the riddle of worm grunting has been solved, apparently. . . .
Kenneth C. Catania of Vanderbilt University has provided an answer: worm grunting mimics the sound of a predator, the eastern American mole, causing the worms to flee topside. . . .
“They come out of the soil as if they are running,” he said. “That is, if an earthworm could run.”_