From Halopedia, the Halo wiki
Bad Dream is the thirtieth track (Disc 2 Track 13) in Halo: Combat Evolved Anniversary: Original Soundtrack.
The track is an arrangement of Trace Amounts from Halo: Original Soundtrack, albeit being shorter in duration. It also places a larger focus on a light synth melody rather than the electronically-generated chimes from Trace Amounts.
The track plays in place of Trace Amounts in Halo: Combat Evolved Anniversary, in the level The Library, when John-117 exits from a tunnel on the second floor of the Library and enters a hallway with blue monitors, up until he reaches the locked door leading into the third floor of the Library's index chamber.
The track also plays in the beginning of the secret eleventh Terminal in Halo: Combat Evolved Anniversary, when 343 Guilty Spark witnesses the destruction of Installation 04 and decides to find refuge at the Threshold gas mine. The track also plays in the fourth Terminal in Halo 2: Anniversary, which depicted the same scene from the eleventh Halo: Combat Evolved Anniversary Terminal.