Operation -- Annihilate! - S1-E30
Trivia: Don't know if its intentional or not? This is the only episode of Star Trek, (the Original Series) that I know of, where a person makes a sound after being stunned by a phaser. As Kirk and crew stun the group of men with pipes, listen carefully as the last one falls to the ground. He makes a "oh-ah" or groaning sound. After reviewing all the episodes prior and after. This is the only person who makes any kind of noise after being stunned. Maybe it was the parasite infecting his body that allowed him to do this.