In STALKER Call of Pripyat...the sound of dogs barking.
"Meh," say those who haven't played a STALKER game, "Why would dogs scare you?" Because in Call of Pripyat, the dogs are pure evil sent straight from hell.
They move extremely quickly and never stop so you can get a clear shot, jump past you when they attack so they end up behind you and you have to keep spinning around, do bleed damage with nearly every hit that slowly leeches away your health, and can take a hell of a lot more rounds of buckshot to the spinal column than a normal dog could, i.e. more than one.
And they travel in packs.
I hate the dogs more than I hate the Bloodsuckers.