I imagine the only people this would terrify more than Gordon is the HECU Marines. I mean imagine it. You're underground, hunting some bastard who is not only seemingly invincible but has already carved a huge swath through trained military personnel, and then these .... these noises start coming from seemingly nowhere.

these are the sounds of developers when they realize they are gonna have to top half life at some point.

This accurately sums up how I feel every time I have to play On a rail.

What if it's Nihilanth trying to instill fear into human survivors?

If I had to guess its maybe from the various portals opening up; consider this: Earth is host to hundreds if not thousands of animal species we only encounter a VERY small number of Xen creatures... who is to say that these noises aren't other horrors from Xen.

This is my alarm...gets me outa bed real fast.

When ever I hear this my eyes start watering. It's probably my inner child being terrified. Seriously though these sounds scare the shit out of me.

This screaming sound also plays at the last portal where you enter chapter nihilanth. I think those are the screams when the unforeseen consequences started

The tentacles make these sounds as they are being burned. They are slowed down, however.

the fact that this goes unexplained is actually insane

I heard one of these sounds are re-used for a cresendo event in L4D1

0:07 "Here they come..."

People , I found out what makes these sounds. These are bullsquids trying to attract the prey.

It's just the scientists having a party! No need for fear!

it's the cries of the damned 

I know what is making these screams, Mr. Friendly.

Earlier this evening, I was walking home and several streetlights had gone out, and I imagined I'd heard this. I was scared, to say the least.

These strikes something deep down inside me. Like, It terrifies me and I don't even know why. It's so out of place in a game and place like this that it triggers a reaction.