Depends on the intensity and distance of startling effect. I tend to react quite strongly to sudden clicking or metal-sliding-on-anything noises, especially up close - in those cases left hand strike at face level with base of palm while getting off the X and right hand on hilt of most accessible weapon at the moment. Due to some... unpleasant experiences with stray dogs, I usually don't stop for a second at grabbing said weapon, rather draw it and advance when I hear the noise of a dog running towards me growling (there were some accidents here involving stray dogs and dead people). If said dog isn't one of the more dangerous kind it backs off - if not...
If anything startles me from behind me I start a diagonal forward leap while turning and evaluating the situation - if I come to the conclusion that everything is OK, the counter-action ends with said leap, if things aren't OK, I start a hammer fist strike with left and grab hilt.
If I'm unarmed... uh, that's only when I'm at home and at home we have "damage inflicting objects" everywhere.
...
So basically I use strikes to create a bit of distance, which is in some cases enough in itself, but would the situation escalate, I rely on armed skills mainly and unarmed secondarily - that is, use the off hand to parry-grab-control-strangulate so it usually ends up open even as in some cases it starts closed. If it was simply some sudden noise with no real meaning... well, I don't wear my knives open so others won't really see what I'm doing with my right hand near my belt and a more-or-less violent unarmed reaction isn't that abnormal so I don't freak them out.