thank you lovely people, appreciate the thoughts!
Poop, thats a very interesting idea, I like it and will keep it for future reference – could be especially handy for suspension pivots etc. Nice.
Trusty – indeed, I have had that before, and I was able to rule this out as an issue this time swapping wheels – same noise
Matty – good ideas, I had not checked that, did a quick pedal swap to rule out issues and noise remained.
In the end, it was very simple, of course – the BB cups, over the last few months of very hard use, had worked themselves a little loose. Quick bit of torquing with with hope BB tool, and…. noise gone!
It was a truly spectacular noise, I guess the BB cups moving a tiny amount in the threads.