From Halopedia, the Halo wiki
Drumrun is the eighteenth piece included in the 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. For the Warthog run, it is designed to make the player feel there is "something else coming" as the piece does not feel like a climax - there are two parts to the Warthog run; part of the Truth and Reconciliation Suite appearing for the second half (essentially an extension of the main Halo Theme). The piece is remastered into Strident in the Halo: Combat Evolved Anniversary: Original Soundtrack.
It can 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.