Going alone in the tunnels, listening some feral ghouls that you canīt see but you know are gonna come sometime for you can scared me sometimes.
Specially playing on a not very good stealth character with not much range for the enemies to appear on your radar (If i do remember correctly there was a S.P.E.C.I.A.L. attribute that affects that didnīt?).
I really dislike closed dark spaces, primary those filled with zombies and alike creatures, both in real life and gaming.
Thereīs too lots of creatures in the game that, without been scary, are pretty creepy. Ants and cockroaches seem very unpleasant to have near, been attacked by or even touch.