nads had a bit of the same thought I had. I've got a MSI Neo Platun (albeit not the same model, as I needed AGP for my gfx card, so the Neo Platnum 2).
In the case of my motherboard, the fan does seem to be thermally controlled. Also Asus comes with software (and there might be options in the BIOS, don't remember) which could give some relief (aka setting a higher thermal tollerance). I don't think I've ever seen mine go over 5k rpm, and seems to gravitate closer to 3,700 to 4,000 rpm, when I've checked
However, depending on how hot your computer is running, if you can add a well placed case fan that can drop case temp a fair amount, the on-board thermal sensors might detect that the northbridge fan doesn't have to run so hot. If your case allows, a nice 120 mm fan, which is well suited (to the assisting the airflow in your comp, based on how your other fans are situated) might help a fair amount. Large fans such as this, if you have room to place them, can get a lot of air flow, without the higher RPM, due to the overall size of the thing...
Perhaps if there are a few dif places you can put this, without unwanted obstruction, some thermal measurements (if you have something to conduct these with) on each placement will help you determine the most effective place to put it for overall case temp reduction...