There are different types of darkness in the Underdark, ranging from harmless to actively malicious. All but the most common type emanate disturbing psychic sounds, but only to those with Darkvision. Surface-dwellers with Darkvision are particularly prone to distress in denser darkness, as they are not used to being blinded by the dark.
CommonDark
Density: Normal - Darkvision, dim & bright light unaffected.
Sound: Silence.
Description: Normal, familiar darkness, like that encountered on the surface.
OnyxStipple
Density: Normal - Darkvision, dim & bright light unaffected.
Sound: Very faint popping sounds, like those that can be made by lips.
Description: Pockets of darkness cling to surfaces, resembling oil on the surface of water. The closer and more intense a light gets, the more these pockets are repelled and merge with the rest of the darkness.
PitchGust
Density: Ultra Dense - Darkvision, dim light down to 0%, bright light down to 25%.
Sound: Hissing, like water touching hot iron.
Description: This darkness occasionally blows through bright light like gusts of wind. For a few seconds, it blows in a random direction while targeting light sources. While blowing, vision is obscured by darkness that resembles black lace over one's eyes.
ShadowScratch
Density: Dense - Darkvision down to 75%, dim light down to 50%, bright light unaffected.
Sound: Stretching or groaning sounds.
Description: Bands of darkness cling to anywhere there is a bump or indent. They distort perception and disguise where handholds or tripping hazards are. As light approaches or gets more intense, the bands get thinner and thinner until they let go of the surface and spring towards the edge of the darkness like a rubber band.
Umbraphillian
Density: Dense - Darkvision down to 75%, dim light down to 50%, bright light unaffected.
Sound: Incomprehensible echoing of feminine whispers.
Description: The edge of the darkness looks like vapor or mist. Thin wisps may gently caress the hands or face of creatures traveling through it before dispersing.