Gabo,
Missed your post earlier.... here is the info:
That is a HP 40x40x20mm PWM Delta fan, and the exact same fan is used in the xw6600 and in the Z600, screwed onto the chipset heatsinks (on the top or on the end, respectively). Most of those fans you'll see on eBay are 3-wire, but this is specifically made by Delta for HP, and it runs just fine from that standard PWM motherboard header on the Z400. You sometimes can find that fan on eBay, but make sure you get the one with the label -7R49 modifier... EFB0412HD, -7R49 down below. Here's a good picture:
Image may be NSFW.
Clik here to view.
Then, I add on a Noctua 4-wire fan speed reducer, the ULNA (the NA-RC6). Those are hard to find, and they reduce the fan speed more than the NA-RC7 (LNA) which can be more easily found in that Noctua made them available in a set of 3 (the NA-SRC7 kit). You can find those on the Noctua web site and buy them from Amazon, etc. Each fan varies a bit, but here are the speeds at full 12VDC, then with the HP PWM motherboard throttle, then with the normal HP PWM throttle and a Noctua LNA added, and finally the same but with a Noctua ULNA instead:
8500, 3745, 3101, 2752
I got a few even higher resistance ones from Noctua in Austria, and thereby you can drop the speed even more and still get good cooling. I think those have 100 and 150 ohm resistors, versus the 50 and 80 ohm ones of the LNA and ULNA.
I've never seen a Z400 with one of those chipset fans attached to that heatsink (other than my mod), so maybe it is really not necessary, but using the processor I am I wanted some added cooling on that chipset because it is handling quite a bit more bandwidth and the regular processors require. That motherboard header was engineered for a purpose, but maybe HP found the big finned heatsink they fit in there was enough. It works great for this purpose, however, and you can see the RPMs in BIOS from that chipset fan just fine. On the motherboard the label is "memory fan" if I recall correctly.
Attachment method: First, the heatsink is attached to the chipset via a glue-type-thermal compound. You'd have to crack it off if you wanted to clean everything and use your own soft compound, like my favorite from Noctua. I got a ruined Z600 motherboard from eBay to see if I could harvest the fan/heatsink from that for the Z400..... can't, but I got to break it off and harvest the fan anyway. For this attachment you see above I took the fan down to Ace hardware and checked out the philips head stainless screws that would fit thru the holes. There is one size that slid in, and one that was just a bit too tight (maybe #6 or 8 ( 8 or 10?). So, I upsized the 4 holes a tiny bit and the larger size then fit. These are machine screws, not the UNC or UNF ones. I got long enough ones to get a bite on the aluminum fins. That has been stable over time. I was going to epoxy on an attachment, but this is working.
Here is a picture with the Noctua ULNA attached, for the xw6600 builds. You don't use as many zip ties for the Z400..... you need more slack. That other fan you partially see is the stock xw6600 memory fan/shroud (those also run unnecessarily fast from HP, so I slow those down with a Noctua NA-RC7 4-wire LNA):
Image may be NSFW.
Clik here to view.
If you look on eBay for EFB0412HD you'll see a bunch of 2- and 3-wire fans but you need the HP PWM 4-wire version one (with the special modifier noted above that you can see on the fan's label). I've only seen a few of those, rarely, on eBay. However, you can find that exact HP 40x20 fan here: http://www.pchub.com/uph/laptop/656-113748-22340/Delta-Electronics-EFB0412HD-Server-Square-Fan.html They last a very long time when you drop their RPMs the way I described.... I have never had one go bad.