휴~ [hue]

If you want to describe the sound itself '휴우' would be ok. And if you want to say it as a verb, it would be '한숨쉬다'. ;)

Both, but we also say 어휴/에휴 to express fatigue, disgust.

haha thanks~! Does 휴 expresses phew or whew or both? From the dictionary: phew (fyōō) Pronunciation Key interj. Used to express relief, fatigue, surprise, or disgust. whew (hwyōō, hwōō) Pronunciation Key interj. Used to express strong emotion, such as relief or amazement.