Tong kosong nyaring bunyinya

Literally: An empty barrel sounds the loudest

Example: “Dia paling banyak bicara di rapat, tapi tong kosong nyaring bunyinya.” — He talks the most in meetings, but empty vessels make the most noise.

English equivalent: Empty vessels make the most noise

Origin: An empty drum or barrel booms when struck, while a full one gives only a dull thud. The Indonesian peribahasa means that those who know least often talk loudest, like English "empty vessels make the most noise."