This is a great question and a situation which I’m guessing most people will have experienced at some stage or other. In many ways, this environment could be a metaphor for our entire life. Whether it is the sniff of a nearby passenger, the toilet seat left up (or down), the cap left off the toothpaste, the person who cuts us up on the road, the dull ache in our lower back, that sensitive tooth, or the rattling and tapping sounds of an antique train carriage—to bring it back to your question—they all act like a mirror, showing us how we feel, what our perception is, and how we are relating to the world around us.
I don’t like to assume anything but my hunch is that there are probably some days when you either don’t notice these things or they at least don’t bother you quite so much. And then there are other days when you notice them quite a bit. And then very occasionally, there may be times when it feels as though nothing else exists in the world. Such is the range of perception. Typically, when the mind is quite happy these things don’t tend to bother us too much. But when we are feeling tired, frustrated or irritable in some way—or maybe even sad or anxious—then these things play on our mind. In fact more accurately, we mentally seek them out. It is as if the mind is looking for more fodder, more gas to fuel the flames of that emotion.
I’m not sure if you’ve seen my TED Talk, but I talk about this exact thing, only in the context of a sore tooth. So we know it hurts every time we run our tongue over it, and yet we keep doing it, every 30 seconds, reminding ourselves of that discomfort and thus keeping alive the story of discomfort in the mind. As I’ve said before, it sounds so funny when we look at it like this, in the cold light of day. And we may well question why the mind would do this to itself. But the mind wants a story, it wants something to keep itself busy and it will do anything to find it, even if it means focusing on the uncomfortable, the unpleasant, or what we might consider to be negative.
So we have a choice: we can either swap the storyline for something positive (often easier said than done), or better still, we can drop the storyline altogether. And this is the practice of meditation, of mindfulness. From what you say it sounds like you are not sitting there witnessing your experience, but rather waiting/fearing/expecting some form of annoyance. And so the mind is thinking ahead, ready to pounce on even the smallest noise. And even when the noise disappears, the mind simply returns to waiting mode, again, ready to pounce.
Needless to say, there is no way of controlling all these things around us, so we have to look inside, at our reaction to it. Because it is the point of resistance, the moment the mind decides it does not like it, which is the problem. So use this opportunity to examine resistance in the mind. Be present with that feeling as the mind recoils or as the body tenses, simply witnessing the experience. And over time, as awareness becomes stronger, so that resistance begins to dissolve. We don’t need to make it happen, it happens quite naturally. But it is only with the focus of awareness, in seeing it more clearly, free of a cluttered mind. In short, less thinking about mindfulness and more simply being present with life as it is.
Hope that’s helpful.