The term 'Sound of Escaping Air' can be summarized by two crossword solutions: HISS (4 letters) and SSS (3 letters). HISS is a sound made when air or gas escapes through a small opening, such as a hissing sound from a punctured tire. SSS is an onomatopoeia for the sound of air escaping quickly, such as the sound of a balloon deflating. Both terms are used to describe the sound of air escaping from a confined space.