I've been wondering about this myself; there seems to be a wide range of reported behaviors on the G5. But I think a lot could have a lot to do with different equipment (video cards, extra drives, DSP cards, etc) either putting extra loads on the general cooling system or adding in their own cooling fans to the mix.
With the infamous G4 Dual mirror door units, the range of reported voulmes was also wide -- but, at least in the recordings they had at g4noise.com, the range of sounds was very broad, ranging from 'regular fan noise' to hair-dryer-with-a-bad-bearing banshee noises; that seemed to suggest that the PS fans in some of those units had serious bearing issues.
Bearing assemblies are like Tolstoy's families: all happy families are alike -- but all unhappy families are unhappy in their own unique ways.
Same with bearing assemblies.
Or how about this... bearings are like apples. One bad apple spoils the barrel...
(Hmm... poor choice of analogies, that last. I meant the fruit kind. Of course, nobody these days has probably ever seen a barrel full of apples, I don't imagine. Pickles, maybe.)