My nearly 4 year old does this too. Mostly with his baby brother but a bit with other kids. He looms right into their faces, juts out his jaw, often makes repeated sounds (e.g. eeh eeh eeh or similar).
I asked the GP's opinion and he agreed it was unusual but also felt he seemed neurotypical in general. There's another thread on here from 2011 that describes similar things and several posters on there said their kids had grown out of it. But of course I guess some kids don't and maybe they will be on the autistic spectrum or in some other way be a bit different, which is fine too of course.
I've never been sure if and how to respond to it, and neither the GP or speech therapist (he also had a stammer though it seems to have gone) had any advice. Now he's got better understanding we usually just ask/remind him to move back and not get in his brother's face, explaining that people like to have personal space and that it's good to communicate with words rather than noises if you can. I'm not sure if it has lessened the behaviour or not, but it feels better to try and help him move towards more successful social interactions if possible.
Like you I have seen other children recoil, and it's hard to see your child get a response like that. I don't think there's anything wrong with trying to help them learn social rules such as personal space - we teach them all the other social rules (i.e. sharing, not interrupting, using the toilet, etc etc) so I can't see how this is different. Maybe some kids just take more help on this one, while other kids have different challenges to overcome.
My own theory on it, if you're interested, is that it's an instinctive non-verbal response to a rush of feeling, like enthusiasm or liking or annoyance. And I think probably as our kids become more verbal as they grow up, they will tend to use words and smiles and other socially-learned responses rather than this one.
My other theory is that for my kid it is related to his on-off stammer, as in both are caused by/related to a tendency to hold tension in the face and upper body. Stammers are said to be neurological rather than psychological, and I feel that my son's looming in and jutting jaw have the same 'feel' as a stammer, as it seems quite automatic and unchosen. But that doesn't mean it can't be unlearned - with stammers the therapist said that's done by avoiding all stammering triggers for a while (I'm simplifying but I think that's basically it) so that the neural pathways that instruct the upper body to hold tension get weakened, while the pathways for fluent speech become stronger. So I feel that by reminding him gently to move away I can gently nudge him towards other responses and strengthen the neural pathways for those.
Sorry for the long reply but it's useful to hear of other people's experiences of this, so thanks for posting. It seems fairly unusual and I've felt a bit alone with it in how best to support my son - whether to try to move him on from it or just let him be. I think whatever happens with it - whether they grow out of it or not - is fine, but of course we are curious about their quirks and keen to help them interact in ways that work out for them. I'd be interested to stay in touch if you like 🙂