There is a deep, cold humming sound with a tinge of crackled buzzing. These sounds were supplemented with a more faint buzzing overhead. The sound of traveling voices peppers this scene.
The source of the cold humming is not organic. The bass of the humming matches the vibration of the floor.
-Chris Williams
Sound Material, Sound Art 
I call dibs on this one!
-Adam
Here is the sound (it’s hard to hear the low tone without headphones):
Edit: Here is the link to the Freesound audio I used:
http://freesound.org/people/niallbrady/sounds/166281/
The crackle was made using presets in Heavyocity’s Damage.
The hum was made in Max/Msp.