The cry of the bloodthirsty demon kittens in Sluggy Freelance: "mew."
Jack has a short arch called "Hell Is that Noise.'' Interestingly since sins speak in a distinctive font, you can recognise the voice of, say, Drip even if you can't see him.
Drip got his NAME from the sound of dripping. Context makes this terrifying.
Again with distinctive fonts representing voices, in Digger the creepy skin lizards, before you learn they are harmless and SGV speak in strange, scary fonts.
And again with distinctive fonts representing creepy voices in webcomics, the Gatekeepers in Schlock Mercenary used to speak in scary fonts, but they don't any more.
If you are on the wrong end of it then “Ommminous Hummm!” counts.
As if that wasn't enough to establish it as Hell Is That Noise to everyone, characters and fandom alike, then came "[S] Equius: Seek the highb100d". After you enter the vent as Nepeta, it's dark, you have no idea where Gamzee is, and all the music stops save for... this. And you can hear the fucking honking. It pretty much made everyone shit their pants.
Even better: That turns out to be the noise Lord English made to announce his birth into the universe. Side note-his purpose is to ensure a universe's destruction.
Used in the most recent chapter of Hanna Is Not a Boy's Name, when they go to visit Tibenoch. They arrived at his house, and started hearing a creepy "tk tok" sound. And then more creepy things happened. And the "tk tok" sounds stayed with you for several strips, until 12:21, when the clock stopped. I'm still not sure what happened at 12:21.
In The Bongcheon-Dong Ghost there's the creaking that you can hear after she takes control of your computer.
Let's not forget the neck snaps. Dear lord, the neck snaps.
