Wolf-whistling

Wolf-whistling or finger whistling is a type of whistling in which fingers are inserted in the mouth to produce a louder and more penetrating tone.

A wolf-whistle is a two-toned sound (like 'whip-woo') commonly made using the above technique to show high interest or approval of something or someone (originally a young girl or woman thought to be physically/sexually attractive). Today, in English-speaking countries, wolf-whistling is considered a form of sexual harassment.