I'm regularly getting this feeling that is sort of like a splash, because as soon as it arises (somewhere near the heart, I think) the entire body is seemingly filled with it. It seems to happen semi-autonomously... that I think, I'm pretty sure I'm thinking of something just before it happens, but it's often difficult to analyze what because the thoughts seem to be near-instantaneous - they must be conscious, though, as (besides the fact that I am conscious at that point) I think they can be lessened if there is an object of focus.
That is, this happens often when I just have my eyes closed and trying to sleep because there's no focus there (for which I've been trying to focus on my breathing)... but it also occurs when a background source of anxiety is too strong for any one focus (and that's virtually everything these days, unfortunately) - e.g. my landlord needed to come here today within an hour's notice, after nearly three years of living here but never dealing with him - I tried to focus on a film afterwards, one that would make most people cheerful, but I kept getting these splashes. This also similarly happens if a neighbour closes a door forcefully etc. which is what makes me wonder whether it can arise from just thoughts.
(Note: I think I called these 'palpitations' in a previous thread here, and the notion kind of fits, but it implies that it's completely autonomous (as if the heart has some difficulties and does it randomly), which can't be true per above.)