As to why some 09 gears are noisier than others, I don't know. It is the kind of thing where a very small difference can make a big difference in noise level. Harmonics, slight differences in how well some pairs mesh, etc.
gear whine is all about load and harmonics, IMHO. More whine at lower RPM's applying throttle to a point, then as the rpm's go up it goes away or simply is drowned out. Either way, I don't worry about it at all. Mine does it in 3rd and 6th.