Applying the principles of my favourite science, physics, I formulated a complex theory on EMI and purchased some clip on ferrite cores. I fitted these to the speaker wire, as close as I could without disturbing anything, minimal effort. Did nothing.
Applying the principles of my favourite emotion, rage, I ripped out all the trim and snapped a few trim clips, pulled a load of cables as far away from each other as possible and taped them down with ally tape. Zero thought applied. Solved it.
I'm now whine free, well, at least until I get to work in the morning.
How does it know to rain when you get the very last hit of trim out? On the plus side, it's much easier to clean and nanolex trim it when it's in a big wet pile in the shed.