I literally cannot listen to this entire track continuously. It creeps me out so much that I have to pause it every so often so I can catch my breath and slow my pounding heart. I've never played Silent Hill 2 and I cannot imagine doing so with any manner of aplomb if THIS is the kind of music I can look forward to.
In SH2, the metal scraping that is Pyramid Head dragging his Great Knife. Even the other monsters flee from it.
The squealing noise (apparently your radio, although you still hear it with the sound effects or radio off) after you leave the hospital in the first game, heard in the track "Over" on the OST.
The air-raid siren that goes off whenever you enter or leave the Dark World. The sound holds extra significance to the Japanese because whenever they went off, any cows within ear-shot would lie down and scream in response.
One of the worst moments in the first game is the trip through the sewers. Your radio doesn't work down there; you have to detect approaching enemies by the noises they make. It doesn't help that the soundtrack in the sewers is full of creepy noises that make it sound like something's coming even when nothing is - distant mechanical groans, odd echoes, splashes that sound like something was right behind you then dove into the water as you were turning to look...
Background noises can get pretty freaky, especially in the Otherworld. The sound of metal scraping across the floor in the next room, the noise of machinery grinding away above you, and other bizarre sounds with no apparent source can get absolutely unnerving really fast.
The Final Boss music of the first game, "My Heaven". It's a nightmarish mixture of a shrill whining, shrieking noise that has been compared to both dentist drills and synthesized baby cries alongside mechanical crashing. It should be noted that this is your radio reacting to the presence of the monster and the radio normally produces nothing more than static, so the kind of presence that would incite that...noise invites even more fear. (Word of God is that it actually is partly a dentist's drill with some processing and pitch effects.)
There's a room in the hospital in the first game where you constantly hear the sound of breaking glass, even though there is nothing at all in the area which could make this noise. There's also the room with the bird cage in Nowhere - you hear a bird flutter madly, even though the cage is empty. And the sound stops once you open the cage.
The periodic hissing sound in the second sewer (heard in "Never Again 2" and "Die" on the OST).
The butterfly apartment in SH2 combines a repetitive slamming noise with the soft flutter of apparently resurrected insect-collection butterflies into a sheer percussive force of horror sound. Neither the slamming noise or the mysterious resurrection of the butterflies are ever explained or even touched on, which makes it much worse.
"Ritual."note You cannot see whatever keeps saying that, but is possible to kill it, and you most likely will want to.
One of the worst has be that squealing noise in the basement in the Otherworld hospital in 2. WHAT THE HELL IS MAKING THAT SOUND?! It's like someone gutting a piglet slowly with a blender.
The worst sounds in the series are arguably the ones that appear without warning or reason. Several empty rooms in the second game sound like there's something huge breathing heavily inside the walls, and an empty sewer passageway in Silent Hill 3 is marked by a woman's scream, running footsteps, and then the sounds of tearing meat. Said scream is also heard during the Nightmare Sequence at the beginning of the game.
The "Black Fairy" music in the alternate hotel, and the music in its basement. Your flashlight and radio stop working, making the ambient music/noises much creepier. Although the radio no longer works, you still hear a static-type background noise in the basement.
There's that freaky music that plays when you're about to go into the Otherworld in 3. And the ambient sound in the underground passage also sounds like the walls are breathing.
The music that plays in the final hallway leading to the inner sanctum of the church is essentially that music with the scare factor turned Up to Eleven. Run.
In 3, there's a mannequin room - only one of them has a head. What's worse is that the room is completely silent save the sound of your footsteps, so you're lulled into a false sense of security. Only when you round the corner do you hear a scream, and return to the sight of a decapitated mannequin, blood included.
For added fun, try getting killed intentionally afterward. Sound familiar?
As though fighting some sort of mannequin head and arms grafted onto a spine and pelvis wasn't unnerving enough, the accompanying soundtrack might just drive you crazy.
4 also has its own share of disturbing noises. One of the most famous collections of them happens in the Building World: there's a pet shop that you visit the first time around, and it's as silent as most rooms. However, when you re-visit the world, there's a gigantic ruckus of dogs and cats in there - even though the shop is empty. After you head out the back door, you can find and read a newspaper regarding a shootout in the Garland Pet Shop - and then you hear things breaking and the animals dying violently, all to the background sound of an automatic firearm shooting almost nonstop. If you enter the pet shop again, it's completely trashed, the wall is full of pockmarks, there's a lot of blood on the floor, and police tape restricts you to only a small passage you can use to go back and forth.
Another one from the Building World: when you are descending a staircase near some elevators, you hear what sounds like a microphone feedback. It comes completely out of the blue and has no clear relation to the plot or the backstory. And it's creepy as all hell.
There is a sound which is shared between proximity to two things: Echo Message locations and Raw Shocks. You always want to run from the latter, but if you want to hear or read the message, you have to force yourself to get closer and closer to the precise location until there's a different audio cue, something moves, the sound falls silent, and there's a button prompt for a new message.
The relief of solving a... "puzzle" is mitigated by a horrible shriek like that of audio feedback.
An interesting and missable example are the radio dedications in Silent Hill: Downpour. It isn't outright stated, but the dedications are apparently an indication of monsters. DJ Bobby Ricks receives calls to his radio station asking him to play songs with dedications to Murphy, which he obligingly does. When Murphy hears these dedications, there are seemingly always monsters nearby. When Bobby finally meets Murphy and begins to discuss his plans to escape Silent Hill, he gets another call for a dedication... for himself. His next line to Murphy is "They're coming."