A moan was a sound made to express pain or annoyance.
In 2372, Quark mistakenly attributed an ear-infection-ridden Rom's moans of pain to annoyance over the Bajoran Time of Cleansing, which had curtailed business at Quark's, saying, "No use moaning about it." (DS9: "Bar Association")
In 2374, Kathryn Janeway recalled hearing the low moaning sound of an injured Cardassian soldier while fighting in the Cardassian border conflict. Her commanding officer, moved by the sound of the poor man, ordered her and an ensign to rescue him. (VOY: "Prey")