Ancient Machine is a song included in the Halo 2 Original Soundtrack. The "Ancient Machine" is thought to refer to different Forerunner constructs.
The song plays on the following levels: Outskirts and the level The Arbiter. It is mainly heard in the level Quarantine Zone. During Outskirts, it plays as you head down the dark hallway in the hotel, and during the short skirmish soon after. During The Arbiter it plays as you clear the hangar. It has some percussion as well as vocals as the lead. It is mostly electronic however, which helps add an eerie atmosphere, which may be a reference to the Flood.