This topic made me chuckle actually, because my cat exhibited his phobia just this morning -- and it's been a long running thing, ever since he was a kitten! He's absolutely petrified of the vacuum cleaner! As soon as I unroll the cord and I plug it in, he starts looking wary, as if something bad is about to happen...but then once it is on, oh boy...he runs as fast as he can into another room, or even upstairs if I am vacuuming downstairs for instance. I think quite honestly it's mainly a loud noise phobia though.
Also, I think he's afraid of being alone too. If we have to go out for the day, and don't get home until very late in the evening, he sometimes won't even touch his food -- I think part of his brain thinks he's been abandoned and will have to ration out his food! Makes me sad when I think about it too much!