Thanks for all the advice on dealing with this! The current status is that as of right now it's worse than ever, we used to live within a few km of a fire station siren and it's roughly that intensity but a continuous sound. It's now very obvious during the day even with the noise from traffic and cicadas so I called the council yet again and finally got someone who could dive into the problem: Since the previous calls were at night, all they could do was send some poor guy out there to confirm yet again that it was a problem, but no action was taken because there was no-one at the factory.
I asked whether they could, you know, maybe call the factory during the day to let them know there was a problem but they said it'd be better if I called noise control again during working hours and they'd send someone out then to have a talk to them. Which means a weekend of high-intensity whine and lots of panadol. But at least the roadblock has been identified and removed, and hopefully some action will get taken.
If that fails I'll try some of the escalation ideas others have suggested, starting with calling and trying to talk to a director who can actually make a decision about it, and then take it from there.