Yes. It's unexpected and creepy.
The sound is more what actually scares you in any horror game though. If you play it on low volume the game won't be half as scary.
And have you seen the original Alien recently? It really isn't as scary as you may remember it.
Who was it on the server I POW Ah HA'd on cp_steel right as the gate opened when he had his sniper dot on my head? That was a pretty sweet tauntkill.