Creepiest noise !
 

Ok, its probably been done before and bit of an odd one this ..what noise/sound makes you frightened/goes through you?.I was a bit freaked out by the strange sounds emitted by the cave dwelling mutants in The Descent and naturally the horrid throaty clicky sound in The Grudge .
How about you?

Long, sharp fingernails on a slate.

Footsteps on a staircase....

The giggling of evil children.

The siren from the Prometheus and Alien trailer.

Actually, this makes me think of another really creepy one: the camera whine from Texas Chainsaw Massacre.

The fucked up sound from The Grudge Dara mentioned is one of the few that really got to me. The background sounds in The Exorcist during the scenes with Regan got under my skin too. Felt like there was a spirit sitting right next to me in the theatre!

Yeah that's a good one. It made the opening so much more vivid.

I would like to say that the creepiest "sound" is actually no sound at all. But the creepiest actual sound would probably have to be the sound of footsteps in the darkened distance.

The echo of the past in the original Woman In Black.