assuming that it is indeed the CPU fan and not the PSU fan, then i'm very aware of this problem. alas, even my system does this, but only once in a while. the problem is usually caused by the system having a CPU fan speed controller built into the board. most major PC venders have this as a feature, but they often times have it controlled in multiple ways.
in your case, its probably a feature built into the mobo drivers in addition to the BIOS having its own built in setting. if you've updated your mainboard drivers (yours would be the nForce drivers from Nvidia), then that is likely the problem. what is happening is that when the computer boots, the BIOS is running the fan at full speed as its default. when the OS boots, and consequentially the mobo drivers, the fan speed then reduces to whatever the driver has them set for as the BIOS will automatically give up control to the OS.
you'll need to check in the BIOS for any kind of setting relating to temperature control or noise control or something similar. just change that setting, and you should be good to go.
just keep two things in mind: you have an AMD board, so you probably have a setting called "Cool N Quiet". directly, this has very little to do with the CPU fan since it controls the CPU speed and not the fan itself. the other thing to bear in mind is that the BIOS could easily NOT have a setting to control the fan speed. a lot of vendors remove BIOS options to "dumb down" the BIOS so that somebody doesn't accidentally do something stupid and destroy the computer.
in that case, you might need to re-flash the default BIOS to get it working right again. i would, however, do my best to avoid that route if at all possible since you could seriously damage the mobo if something goes wrong.
anyways, hope that helps.