Now I can hear it better I would say it's a bearing, the noise is not uniform or consistent, it is more like a rumble. It is a crank bearing because of the frequency but it could be a crank bearing, or even the belt pulley wheel bearing. Both sprag bearings are locked while this is happening so we can rule those out. The crank bearings are relatively rare, so I would guess the inner crank bearing or large pulley bearing.
It's not something I have heard before, but then I don't normally run the motor on a stand. My guess could be totally wrong, but it would be worth investigating. Drop the crank arm off, can you feel any notchy or stiff feeling through the crankshaft?