Internal noise is what distracts me most. Sometimes I lose focus because I’m thinking of all the things going on that day or all my problems. If I have a song stuck in my head, that can distract me too. I usually get distracted by one thought, and then I start thinking about a bunch of other things, and then I’m not even paying attention anymore!
External noise can get me too, mostly if someone is fidgeting next to me, or if they’re doodling on themselves or a piece of paper. It is also so hard to listen when there are other conversations going on at the same time!
If you’re in a one on one conversation and you get distracted by noise, I think it’s best to just tell them that you weren’t listening, and to ask them to repeat what they said because just guessing at what you thought they said can be dangerous (I speak from personal experience). If it’s in a larger setting, I guess I mostly just try my hardest to focus, although sometimes I still get distracted.