I have two cats and one is much more nervous than the other. Any noise sends her scurrying under the couch or the bed. If I turn on the vacuum, she’s gone!  Nervous cats are fearful which prevents them from discovering that the noise was actually harmless. Some cats are more fearful than others which could be something genetically related. However, the majority of nervous cats are those that have had a lack of experience with people or new situations during a crucial time in their development. My younger cat, the nervous one was adopted when she was four months old. She was abandoned and I suspect faced a very fearful situation which makes her frightened all the time as an adult cat. Even though, I do everything to try to make her feel calm, any sudden noise will send her running. Sometimes she stays in her cat bed with her head down