The Lost Song
From Halopedia, the Halo wiki
The name The Lost Song is a reference that it was "lost" from the soundtrack. The track features slow percussive beats and high strings playing a suspenseful melody with bass guitar in the background.
The track plays during the level Assault on the Control Room right before the bridge that overlooks the Control Room. After crossing the the two bridges with the two Type-26 Banshees and Mgalekgolo, the player enters another large room. After eliminating the Covenant in this room, the track will start playing. It plays on an infinite loop until the player proceeds to the hallway and large room after.
The track was arranged into Arborea Above in Halo: Combat Evolved Anniversary in Halo: Combat Evolved Anniversary: Original Soundtrack.