It's really common.
dd2 was exactly the same, and even used to fall over if a lorry drove past because of the noise.
sudden unexpected (and even some expected) noises would make her Moro kick in and she would crumple. (Athetoid cp)
We did continue to work on it though, and it lasted about four years - at 9 now she can cope if they go off without warning - she still startles and puts her hands to her ears, but doesn't drop to the ground or cry.
She is still very noise sensitive, but we've worked very hard to desensitize her - we want her to be able to function - as an older child in a mainstream environment, she needs to learn coping strategies to deal with 'ordinary' life... Which includes hand dryers, magic flushing toilets (the terror), fire engines, speakers on aeroplanes, speakers in theaters where you can't see where the noise is coming from and that spring to life in the dark, story tapes, orchestras... The list is pretty much endless where sudden noises are concerned!!
This year she even took part in the town parade, where we were placed directly in front of the fire engine, which honked, wailed and generally made her life an absolute misery. But by god, she did it.
even if it was with her hands over her ears, which made walking really difficult. 
We did use the disabled loos when we didn't have time to do the whole recce/ prep/ lengthy visit and calm down routine though. 