Noises and unexpected events are a part of everyday life, but the autistic brain struggles to process this properly. This results in the trait described as "Inappropriate response to sound or no response to sound". Usually, noises are so overwhelming that I don't respond at all. When it comes to being startled physically, however, that's a different story. Some got a good laugh from me getting surprised from behind. I'm sure they thought I was exaggerating a little, but I wasn't. I truly do "overreact" to being surprised.
I tried to warn a boy once not to do anything unexpected. Several times, actually. I guess he got carried away in whatever he thought was going to happen, and attempted some sort of unexpected gesture. I punched him in the face. When people vent, they often talk about how satisfying it would be to do that to someone. I must say, it was not as satisfying as they said it would be. It was a reflex and not of any fault of my own. I felt bad, but I did warn him.
When I was a kid, loud sudden noises made me want to cover my ears. Doing this brought attention to me, so I stopped. To be completely honest, to some extent I can relate to autistics who bang their heads against a wall. Sensory sensitivities are painful! Seeing a non-verbal person have a meltdown looks like a more physical version of my own. Like an unrestrained version of how I sometimes feel.
I've experienced moments where something startled me that wouldn't even be noticed or understood by a neurotypical: fast approaching cars, people approaching me spontaneously, tv or radio commercials, and even seeing familiar faces in different lighting. To an outsider looking in, it might very well appear as though I'm on drugs. My brain registers the slightest changes and movements, meaning I live my life in high resolution all the time. I startle easily, and my reaction may startle you. Boo! ;)