Possible Clues:

• It's often played in the dark
• Sound in some sleep apps
• Sound used as a sleep aid
• *Background sound that may be a sleep aid
• Concentration aid
• Knock one back in discerning frequency that one associates with a fan
• Broadcast I see, on with random sound
• 1985 National Book Award winner about an "airborne toxic event"
• Classic DeLillo novel
• Sleep aid, for some