Took me about two seconds to come to this decision.
Take Call of Cthulhu: Dark Corners of the Earth, either on the PC or the Xbox, doesn't matter.
There's a part where you have to go through some sewers in Innsmouth (bad formula already), and you know that a shoggoth is down there. You have no idea what it looks like, or what it sounds like, or anything.
When you get close to it, you enter some kind of pumping station under the city. I don't remember what it was supposed to be pumping, only that the machinery was loud. So loud that the machines drowned out the sound of the shoggoth.
I spent a solid four minutes with my back against the corner of a room, shotgun in hand, waiting in breathless anticipation of some horrible death. Maybe I thought hte machinery's sound was the shoggoth, instead. But that was the scariest sound I have ever heard in a game.