Our cat is 3 years old, and for the last year and a half or so, she became very stressed out regarding any unexpected little noise. For instance, grazing a carboard, coughing, the creasing of a paper, even the sound of fabric when crossing legs sometimes. She will either just jump, or, more often, run away as quickly as possible. But she will also calm down in a matter of seconds.
But she isn't afraid of noise per se. She doesn't care that much about a vacuum cleaner noise, loud music, lot of unknow people talking loudly in the room (in which case she'll come and stay aroud us). Even when se travel, with her in a bag, and in loud places (train stations for instance), she is rather calm, and more curious than wary.
We live in a rather calm place, and we never heard any external noise that seemed to scare her. We both work, and when we come back home she is usually rolling on the floor asking to be pet. She isn't a very cuddly cat, but can spend dozens of minutes on our lap, purring (especially when she is hungry). She also loves to play, although we could play with her more (we both work, and we play with her for a few minutes most of evenings).
The only stress-related event that happened to her was one time another cat that came to her territory (a rooftop terrace of a 50m² flat). It lasted a few weeks, maybe 2 years ago, but his owner was comprehensive of the situation and prevented him to wander there.
So, apart from these noises that make her very stressed out for a few seconds, she seems perfectly happy. Any idea what could be causing this, and what we could do to make her more serene ?