Well I'm still rather young, and headcrab zombies (mostly fast and poison) creep me out after reading the TvTrope nightmare fuel article on HL2.
I mean seriously, the poison headcrab zombie sometimes laughs when it dies.
It's happy that you killed it.
And the alert calls that the fast zombies make (eeUUURAAURAAURRRrr) creep me out, especialy while watching them on a rooftop, doing their 'taunt' and then scrambling down a drain pipe to rip my organs out.
Oh, and the sound normal zombies make when they are on fire.
They sound like a human instead of their slightly distorted moans.
And if you didn't know already, the human they control is still alive.
;-;