EDIT: I realised too late you were talking about PSU fan replacement, and already read posts I was going to addAny chance those pics have been posted yet? I have fans in need of replacing
I'll leave the following as a summary for newcomers regarding normal fan replacement and some links to fan mods.
To do a like-for-like replacement of a fan is super easy. The entire fan tray simply unscrews in the same way as a PSU does - two thumb-screws at the back, then pull it out:
Each fan is then held in with two diagonally-opposite screws into the back plate of the fan tray.
I've not yet replaced one myself, but one of mine is intermittently failing so I might do soon. The 4-pin connector is a different wiring to standard, so this needs to be checked for (see below.)
Here's a close-up of a fan:
By AVC, model number DB04028B12U. I can see a few on eBay - there's some cheap ones from China, but they're only 3-pin. But apparently the same fan is used in an HP ProLiant model, and there's some of those - albeit rather expensive.
Best of all is to put in one or more quieter fans, as others have done. Again, be aware that the 4-pin wiring is different to a normal fan, potentially requiring swapping pins on the connectors.
Here's a post from a guy who's done a single-12cm-on-the-top mod.
And here's Sleyk's mega-post on doing a fan mod, including lots of photos and discussion of the different 4-pin wiring.