Sentinel launcher
From Halopedia, the Halo wiki
Summary
Sentinel Launchers are small devices built into the walls of some Forerunner facilities. They are protected by armored covers that dispense Sentinels when hostile entitities, such as Flood, are detected in the immediate area. One way to destroy a launcher is to attack it when the armored cover opens to release a Sentinel. It is also possible to destroy a closed dispenser by sticking a Plasma Grenade on its cover or meleeing it although this will cause your shields to become depleted.
