A sentry is shown hitting a gong.
A gong is defined as a metallic disc, circular in shape, that produces a resonant sound upon being struck. The Bahk-tov Council of Bardotta utilized the sound of a gong to mark both the beginning and the end of their sessions.
On the forested planet of Corvus, Morgan Elsbeth's army employed the gong as a signal to raise an alarm. Specifically, during the events of the liberation of Calodan which took place in 9 ABY, Elsbeth's forces sounded the gong as a warning when the Force-sensitive warrior Ahsoka Tano made her appearance to free the city. While fighting the guards of Calodan, Tano used her lightsabers to cleave the gong in two at the city's entrance.
The initial appearance of gongs occurred in the Star Wars: The Clone Wars episode "The Disappeared, Part I" which aired in 2014.