From Halopedia, the Halo wiki
Drumrun is the eighteenth track in Halo: Original Soundtrack.
The piece gets its name from being composed almost entirely of drums (there are a few synth sounds in the background, possibly representing the Flood), and that there is a sense of urgency or impending danger.
It can be heard in The Maw. It first plays when John-117 comes across a locked door being forcibly opened by the Flood before entering the corridor leading to the cafeteria of UNSC Pillar of Autumn. It plays a second time after he exits the bridge and encounters a group of Flood forms exiting at the door to the armory. It plays a third time at the start of the Warthog run at the end of the level.
The piece was arranged into Strident in Halo: Combat Evolved Anniversary: Original Soundtrack.