Read this. I dare you.

In the late 1950's, scientists accidental discovered something that isn't meant to exist. Since then it has been dubbed "the frequency".
The frequency works on a basis that prolonged exposure increases the potency of the effects, for example, somebody who was exposed for ten minutes would have less effects than a person exposed with one hour.

The frequency remains largely a secret, as the reveal of it, it's effects and it's origin are likely to cause panic, we'll get to that at the end.

Naturally, The scientists responsible for the frequency felt it would be best to test it, the first test (lasting 20 minutes) showed no immediate affect other than some acute anxiety, test 2 was different.......

it lasted an hour long, shortly after the subject complained about fog, in front of his eyes, and shadows coming towards him out of the fog, he then picked up a scalpel and grabbed the scientist nearest to him, slitting his throat before taking the scalpel to his own eyes, attempting to rid himself of images.

Naturally, the tests were scrapped and the research was buried in the archives.

Anyway, the important part, I put your life at risk by waiting this long to tell you. This is how to identify the sound, it is a low and loud hum, upon hearing it you must get away, fast! Although it's probably too late, considering the time you've been on this site, trying to scare yourself. You're soon to experience what real fear is.

Let's review the facts, it is generated in a place which would cause mass panic if discovered, makes you feel fear and anxiety, and makes a low, loud hum.

All seems pretty damn inconclusive to me, but wait, what's that sound coming from your computer?