Oh I'm so sorry. I have similar but it's meltdown related (started age 7, now 14. Less often but now louder and with added swearing - great!)
I ended up writing letters to all of the neighbours in our block of flats (I don't think you need to talk to anyone other than your below neighbours thankfully).
I explained that one of my children was autistic, explained about sensory overload and meltdowns, apologised a lot, explained that it was largely beyond their control, (it was probably around twice a week but was pretty bad at one point) and enclosed information from the NAS.
I find flat living odd anyway as you never really get to know everyone in the block well enough to be friends, but it kept the complaints at bay. I don't think my immediate neighbours are ever going to like us much (they helpfully 'suggested' that my kid would be better off in a 'home' of some sort) but I know I've done all I can.
I know it's worrying, but actually, you can't do any more than take any practical steps, and explain to your neighbours and hope they understand or at least can point out if there are any times of day when it matters less.
Sadly we can't afford to move, but as a friend said, if they keep complaining knowing that you have a disabled child and that's the reason for it, then they're not that nice. Good luck.