This question made me smile. Cats can be quite silly and seemingly random when it comes to things that scare them. My cat is afraid of the vacuum cleaner and toilet when it flushes. It's pretty funny with the latter because he always wants to come into the bathroom and whenever I relent and let him in, he sits there purring and sometimes headbutting for pets. However, if I flush the toilet, he dashes out of there like a madman. I guess it's the sound.
He also acts very timid when there are thunderstorms. Like my old dog, he goes all quiet then slowly slinks to under a table or some cluttered corner to hide. All the petting and reassurance doesn't seem to comfort him, and he just wants to be alone. Once the storm is over, he's back to his normal, bossy self as if he wasn't a total coward five minutes ago.
