I absolutely agree with amarmai that "There is a pattern emerging here of single older men who have lost the opportunity to exercise power at jobs, over wife and cc and seek to satisfy their need to be in charge over neighbors who are single women with children."
OP, I think you need to STOP worrying about noise. It is absolutely not bothering him, it is just a convenient excuse he uses to crowbar himself into your head.
A couple of things you've posted:
"I struggle to be assertive" - and he's exploiting that. I think if you work on it, practice it, you'll find assertiveness will be your best friend.
"I've tried the friendly tact and he chatted me up!!" Eww. But useful, because it shows how he thinks. And confirms that friendly is NOT the way to go.
"New neighbour comes out of his apartment everytime I do. So if I go to the shop, work, bins etc he comes out. He always does it passive aggressively on the pretense of just bumping into me but it is everytime I leave the house and he always makes some comment of being able to hear the kids walking around, he brought his boys up to tiptoe etc. He has followed me round the shop before now."
This is, I'm afraid, where you would want to be assertive. He is depending on you being polite, and bullyable. So you need to be impolite and hard-faced. I know, it will take practice.
So when he comes out, say something along the lines of 'What a surprise, I leave the house, and you appear. You do realise it's very noticable, and no, it's not 'just bumping into me' it's deliberate. Stalking, in fact.' And I would deliberately use the word 'stalking'. Any reference to noise, cut him short, don't even let him finish the sentence just talk over him. 'Any noise from our flat is normal, if it's such a problem go to the council and get them on to me, until you do I don't want to hear about it any more.'
Remember - THERE IS NO NOISE PROBLEM. There really isn't. I'm so sorry he's stressing you out, but he will only stop when you stand up to his behaviour. You need to call him on it, ASAP.