Here's a recent article I found called "Genes effect anxiety and startle response"
http://talentdevelop.com/articlelive/articles/761/1/Genes-affect-anxiety-and-startle-response/Page1.html
I'd always wondered that also. I felt like my startling so easily were related to my anxiety.
Ive read sisnce that it can be caused by high norepinephrine levels from havinf to low of cortisol levels.
You do not mention if you take any medication....noise sensitivity is a side effect of many meds.....I have always had a very high "startle response" myself,so I feel you..when I took Celexa,an anti-depressant,that made my noise tolerance pretty much zippo,but I think that a sensitivity to being startled is just part of some people's basic makeup
JEEZ this happens to me when im sleeping, i will just jump out of bed cause im startled by something, and my heart races, but noone is there to startle me....If it bothers you alot you should go to your doctor and tell her whats going on. But its normal for people to jump to a sudden loud noise. Its your noise reaction.