My cats are pretty stereotypically afraid of my vacuum. As soon as they see me getting ready to use it, they'll jump up. The second it gets turned on, they're bolting to the other room.
I watch movies and play video games that make sudden loud noises, so it doesn't seem likely to me that the vacuum is scary because it's loud. Is there something else about the noise that's scary? I was thinking maybe the frequency was at a certain level that only bothers cats, like how humans can't hear dog whistles.
What is it about vacuums that are so scary?