Stewart,
Interesting. I sure don't know what is going on. If your clutch is adjusted properly, and fully breaks the connection between engine and transmission, then it sure sounds like it IS the throwout bearing making the noise.
I'm not a believer in the 'bad off the shelf' philosophy. I haven't replaced gobs of throwout bearings, but have never found a new one that was bad.
You have replaced two, and still have the same problem. The only other thing that comes to mind would be clutch fork rubbing on something, but that's a stretch.
If for some reason it is a throwout bearing, you should be able to depress the clutch pedal only enough to take the free play out, and the bearing should start to growl. Keep us posted on the head scratcher.
Stuart
