For example, a part I just went through: you have to climb into a blood smeared vent to progress. As you climb in, you hear something in the vent somewhere...
sniffing. Not breathing. Sniffing. Constantly. Always near by but too far away to really pinpoint it.
Keep moving forward and half a zombie will drop down through a vent and attack you. You might think that's the source of the sniffing, but...no. The sniffing's still going, and if anything, it's louder.
Keep going--until you hit a dead end. Something smacks you in the back of the head; you turn around--and the vent you just went through? Is gone. The walls are, very literally, closing in on you.
After letting you run around through the ever-changing, always-closed off vents like a rat in a maze for a while, the game suddenly drops you down into a dark, dream-like room where all you can see is the occasional glimpse of a wall via your flashlight. No exits, no
anything. Except for the sniffing. Getting louder and louder and mingling with some animal sound, like something's hunting for you--
...And just about the time all that's got you backed into the nearest corner, swinging your axe wildly and waiting for whatever it is to come charging for your face, you get dropped back into the hospital. Like nothing ever happened.