Drumrun is a piece included in the Halo: Original Soundtrack.
The Maw is the only level in Halo: Combat Evolved in which it is heard. It appears several times briefly, usually when John-117 is dealing with new waves of enemies. Most notably however, it plays during the first part of the Warthog run at the end of the level.
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 or 343 Guilty Spark), 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 (essentially an extension of the main Halo theme).