Why do people make a "koi" sound at the cinema theatres??

Went to the pvr cinemas in forum mall a few days ago in Kochi and heard some guys making some ridiculous noises like "koi". What does it mean? What's the point of it ? I'm from the UK so I have no idea what's normal here. The sound was the most ridiculous thing I've ever heard lmao. Not sure how they aren't embarrassing themselves.