I'm reading this with interest because Angus is going through this stage atm. There doesn't seem to be anything malicious about it, sort of a frustration thing. He's very gentle with the babies and gives them kisses. It's me or DH or the table/cup/wall/bowl/toy he hits. Once he hurt me and i drew a sharp breath, and now he sometimes hits me then makes that noise. It's like when he throws something on the floor then says "nooo" or "ooh, noo".

I don't think it's something he's learned by example, it seems to be just a stage, and I find the best thing to do is play it right down. Stop him from doing it and look away, not give attention for it, and give lots of attention for nice things like kisses and cuddles and clapping and dancing etc. We use the word 'gentle' a lot too when patting the pets or babies gently.

If you have to do something because other people or children are involved, I'd suggest physically stopping the child from doing it and telling them we don't do that in a firm but downplayed way, and then serious distraction to the best of your ability in a way that doesn't reward the behaviour, easier said than done I know.