On replacing the PSU fans, see
here.
If you haven't done so already, try cleaning the fans, it can make a big difference.
I bought a used dual 867 which had the Delta kit and was loud as heck. I also bought a brand new MDD case (service part) that's my 1.25 Dual; this new PSU had Nidec fans in the PSU and the 120mm Pabst fan -- the new case is much quieter.
Based on that above blog I linked in both PSUs I swapped the fans with Akasa thermal sensing fans -- it's what I could find along with some other stuff I needed for the MDDs. I found the Thermaltakes mentioned but they didn't other stuff I needed -- shipping can be killer if you have to buy from so many sources. I probably didn't need to swap the fans in the new PSU, but while I was at it I figured I'd go ahead and do them both.
I think the PSU fans can be loud -- it's a pain, but best way to know is to open the PSU and see exactly what's inside. Check the specs. Apple used a number of different 60mm fans in the PSUs and some are louder than others, so at least you can establish some sort of base line.
Swapping the Akasa for the Deltas in the 400W PSU in the 867 made a huge difference -- the Deltas were serious hair-dryers on Max. But in my used 867 the 120mm Delta was awful (loud with shot bearings). Once I swapped that for the Pabst 120mm offered as part of the PSU exchange program (a friend of mine had some MDD parts in his garage he sold me for cheap), that case is much better.
Of the 4 fans (2xPSU, 120mm case, 60mm optical), the PSU and case fans made the most noice. The little optical fan can make a bit of racket too -- might be worth cleaning out the dust. With that one, you can easily unplug it temporarily (without huge risks to cooling) to see if that one's making a bunch of noise too. You can do that too with the big 120mm case fan, but don't run it for long at all. Run it for a minute or two just see if you can identify the source of the most fan noise.
In my 867 I replaced the optical fan too -- since even that Delta was loud. New fan made too much noise for my taste, so I'm powering it only 5V and it's not plugged into the mb.
