zviždati

1. to whistle
   Moj deda uvek zviždi dok radi u bašti. — My grandpa always whistles while he works in the garden.
   Sudija je zviždao, ali igrači nisu stali. — The referee was whistling, but the players didn't stop.
2. (+ dat.) to boo; to whistle at in disapproval
   Navijači su zviždali igračima posle poraza. — The fans booed the players after the defeat.
3. (of wind, bullets) to whistle; to whizz
   Cele noći je vetar zviždao kroz prozor. — The wind whistled through the window all night.

In Serbia, whistling from an audience usually signals disapproval, not approval.