It appears to be the Northbridge fan. I have a Gigabit board and it runs fine without the fan, however, if you choose to remove it be sure to monitor your temps closely. If you want to replace it, I have had good luck with this company:
http://www.3dcool.com/Products.asp?id=74
Normally, the 40mm fans come in two thicknessnes. Just make sure you get the right one; also, determine the number of pins that the current fan uses. If it has a 3 pin connector then it has speed sensing.
Another option is to replace the fan with a heatsink. Sometimes they're a bitch to install depending on the motherboard.