Most dreaded sounds in gaming.

[Thrane]

Nydus worm popping out of the ground can be rather dreadful too. Since you can hear it no matter if you see it or not so even if he just connects his bases you think there's an army inbound.
Not that many people use 'em though so guess it's not as dreaded as a nuke atm.

 

[Spiner909]

Strategic Launch Detected.

 

[bad_dog14]

The cougar in Red Dead Redemption and the zombies in Half Life 2 that have the black headcrabs on them *shudder*

 

[ArchBlade]

Lot's of stuff.

The tank's theme.
The clicking of a claymore on CoD4/MW2.
The spring noise a betty makes on CoD:WaW
The sound of a grenade hitting the floor in any game that has grenades.
The howl of a cougar in Red Dead Redemption.

Stuff like that.

Also, for me personally, I'm really coming to dread the snapping sounds the boy from LIMBO makes on death. Normally I wouldn't be, but when you're trying not to die more than five times... Well, suffice it to say that's a pretty scary sound.

 

[TheGameXXVII]

Tank music from L4D/2
i'm just walking along then all of a sudden...
DUM,DADADUMMM,DUM,DADADUMM...then i go "OH SHIT!!!" and run like hell, spraying bullets at it in hopes that it's drops dead before it gets to me.

 

[scoHish]

Whenever I hear one of these bastards breathing somewhere in the dark I pee a little.

 

[cefm]

Snake! Snake! SNAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAKE!

 

[seanez]

The wonderful grating of a hard-drive ripping its own guts out

Oh, in-game? Probably HL2 fast Zombie howl. The critters might drop to a double shotgun blast to the face, but their howl puts me on edge. Nothing much else comes close - kudos to that sound engineer!

And to various other people quoting this. The sound is called "The Wilhelm Scream"
It's in the nutri-grain add

Also that sound, spider sounds and Re-deads/Gibdos.

Er, sorry to burst your bubble but I don't think it is. The Wilhelm scream is that really annoying crappy one that sound guys seem to bum:

Here's the HL2 fast zombie one, being analysed a bit:

My bad its the "ironman scream"

 

[ProfessorLayton]

Spoiler: Resident Evil 4 had this gem.

I like to do this to my friends while they sleep. It's... delightfully entertaining.

I hate you for reminding me about that.

But anyone who's played Metal Gear Solid... any of them... should say the ! because not only is it a sharp and startling sound, it means you've got to deal with whatever guard saw you. It's especially bad when you're trying to sneak away and it's completely silent.

 

[AngryMongoose]

That sound used to fucking TERRIFY me as a kid. Especially since I was usually taking the linking book to the fucking mechanical age, the only book I knew how to access.
The rogue stealth/stun sound effects in WoW tend to make me freeze up, since that's what my character is doing.

Also, I know it's not from a game, but since the main source of the SFX for me was the disc for Theme Park, I'll add it. The sound effect that used to indicate a hardware failure on a mac used to make me feel sick. I have LITERALLY hate nightmares about the icon that comes up. When I heard it at the age of 14 for the first time since I was 7, I froze, my heart sank and my entire body went into "OH GOD I'M GOING TO DIE" mode.

If anyone can link my to that sound effect I'd appretiate it; can't find it myself.

 

[Wicky_42]

...
My bad its the "ironman scream"

Ok, wow, I didn't think it was possible to get a worse stock scream that the Wilhelm, but I think that does it XD Sooo bad - it doesn't even fit anywhere! Well, other than as the HL2 zombie sound - works pretty well there.

 

[Nvv]

not a huge fan of it but the old *click* BOOM claymores on CoDmw1,2 are quite bad

The funny thing with that is the timing. You hear the click, but it doesn't go off. It goes off just after you've been able to register the click and think "oh fuck".

 

[Life_Is_A_Mess]

"Hey!... Hey!... Hey!... Hey Listen!... Hey!..."

Sentenced to be hung until death.

 

[Life_Is_A_Mess]

Strategic Launch Detected.

And this one...

 

[Captain Pirate]

The little "Crack! Thhhhhhhhttt..... ...Whfump." of a sniper bullet being fired from a distance at you in BFBC2. And killing you.

 

[Drop_D-Bombshell]

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He scares the hell out of me even today!

 

[bew11]

The clinkity clink clink of a grenade bouncing toward your feet.

 

[John47]

the sound when a fast zombie in hl2 is climbing a pipe it's like this "clang" "clang" "clang" for about half a minute then there's the tank theme it's creeps me out so much when i hear it
because getting close to brutish enemies in shooters and hack and shlash games