Power core
From Halopedia, the Halo wiki
Power Cores are very similar to Fusion Coils, Gas containers and Plasma Batteries. Their purpose may be to store plasma energy to power up the Forerunner machines and proved to be very fatal when exploded. Power cores can be found mounted on walls in the Campaign Levels as well as in some of Halo 3's Multiplayer maps. They are recognized by their low pulsating humming. These can be used to kill anyone who passes by, if they are shot at. However, it takes two Power cores explosions combined to kill somebody without shields. In Halo 3, certain maps can spawn the Power Core in Forge.