はらはら
はらはら · harahara · JLPT N1 · noun
feel nervous
hara = 腹 belly; doubled, it mimics a gut fluttering with nerves
The same doubled sound describes leaves fluttering down (はらはら散る). Nerves feel like a fluttering stomach, so the mimetic fits tense waiting, like watching a close race or a tricky moment.