jamesinc;51193 wroteI had a poke around and now I believe there are two separate noises in play. The throw out bearing is definitely one as when I pull the clutch fork back it goes away.
The other sounds similar but is louder and kicks in near 3000rpm and is THE ALTERNATOR.
Great research work.
Focusing now on the whirling throwout bearing - What is pre-loading this is the clutch slave cylinder - specifically the central rod then touches/engages the clutch fork. By varying the rod length a little (few mm) can have a small effect on pre-load. Older systems had a thread on the end of the rod and you adjusted the half moon nut, but nowadays when you buy a new slave its most likely supplied with a fixed rod length.
I have tried a spring to try and pull back the fork, with no effect as the slave cyl has no-play, hence the reason to focus on the rod length = a little bit shorter.
The other aspect is the clutch pressure plate -> the fingers. The finger angles and curves vary a lot between manufacturers. The last genuine Volvo (=SACHS) one I installed the fingers curved out a long way, and so the chance of the thowout bearing rubbing more than its should = most likely. It was also stiffer that usual - as in more foot force required.