My chipset fan just went out today. It was still spinning around 600 rpm, but was making quite a racket. A quick call to Asus tech support and I have a replacement on the way, but what a pain to replace.
A quick and probably temporary fix I found was to remove the fan from the heatsink (four screws on the top and 3 below the blades) and blow all the dust out of it. The blade assembly will lift off its base a bit to help out. I put it back in and it's back up to 8000 rpm and will hopefully last until the replacement arrives. I don't know exactly what caused the noise. Was it dust inside the motor or was the motor just a little loose and vibrating within the heatsink? At any rate, this method is working at the moment (been running about 2 hours).
Now, I don't really want to remove the motherboard and replace the whole assembly if I don't have to, so I looked into replacing just the fan which is a 10 minute job. I googled the part number on the bottom of the fan and found a place that sells it. I'll let you know how much when I hear back from them. I'll probably buy a few of them and replace as needed........much easier than taking the whole computer apart to install the one Asus sends me.
Update: The part is not available.........