My mac pro noise reason establishing and fixing some.
My new 2.26 octad mac pro was very noisy.
1. Massive low-pitched fridge-like vibrations - very annoying;
2. Optical drive low- and mid-pitched vibrations and high-pitched resonances;
3. GT120 mid-high-pithed hum - rather loud, but not annoying due to uniformness.
4. PSU buzz and purling (like electrical breakdown or electrical arc) - rather quiet.
As for 3. Obviously it can be fixed by replacement videocard by quieter one (they say some flashed 8800GTX are most quiet macpro's videocards ever - any suggestions or variants?).
As for 4. I noticed that it depends on power cable spatial positioning. In several positions buzz and purling are almost gone.
As for 1 and 2. I noticed that fridge vibrations problem relates to fans, mostly to intake and exhaust. And it is possible to lower it by setting fan's speed to some values. However I stopped these fans mechanically but fridge-noise left. Likely this hum problem depends on intake and exhaust fans frequences, but relates to whole system "Case <-> All of the fans". This system have it's own resonance frequences. So I tried to find places which resonate heavier. I inspected mac pro and noticed couple of crappish chinese assembling ****ups:
1). retention bar and pci fan was not fixed.
2). (most horrible) As you know, optical drive bay has slits on its bottom which have to be slided along the screws. But in my case optical drive bay's slits was not putted on screws. Instead of this it was placed over screws. and pushed in by brute force. In place it was not fixed and was vibrating. But it was very hard to take it out because it became jammed if I tried to pull it out.
After fixing 1 and 2 all vibrations, resonances and low-pitched noises became quieter, but have not gone. Tried some smcFanControlling (Intake ~700 and Exhaust ~650, not remember exactly, I'm at other computer now) and it have almost gone (now it is uniform, very quiet and almost unperceptible like healthy mac pro).
The most loud part of my mac pro now is GT120 fan noise.