From Halopedia, the Halo wiki
Dread Intrusion is the eleventh track (Disc 1 Track 11) included in Halo 3: Original Soundtrack, and plays throughout the Halo 3 campaign level Floodgate. It is composed of Dread Intrusion (0:00-1:50), Black Tide (1:50-3:43), and Old Friend (3:43-5:25).
Dread Intrusion (1:50) begins with digital chimes and sound effects. Muted percussion enters, leading into a reversed message hidden in the piece, then transitions with digital chimes and sounds. The opening ambiance is played in the beginning of the level, and the reversed message section plays when John-117 and Thel 'Vadam enter the Flood-infested Traxus Factory Complex 09.
Black Tide (1:53) enters with female vocals singing three phrases in eerily. Percussion enters, followed by vocals and low strings in a reorchestration of the second section of Ghosts of Reach (from Halo 2: Original Soundtrack). This piece plays in the chapter "Shadow of Intent", when John-117 and 'Vadam return to the second lake bed to witness the arrival of Fleet of Retribution and Sangheili soldiers being deployed to combat the Flood. The vocals were invoked in Once More Unto the Breach (from The Pillar of Autumn in Halo: Reach Original Soundtrack).
Old Friend (1:42) enters with eerie ambiance, mixed with various digital chimes. The notes play in no particular melody before they transition softly into high strings. The piece plays when John-117 enters the wreckage of Indulgence of Conviction in search of Cortana and, as the title alludes, meets 343 Guilty Spark.