Fan keeps Reving need help!

I just built my rig today and one of the case fans i have keeps revving, i can see it in the bios but my question is how do i fix it? 

Revs randomly or just on boot?

IT could be that the temp sensor that is associated with that fan header is bad, or the fan is bad, or you have the fan plugged into the wrong fan header so the PC runs the fan harder when a different region of the PC is heating up. also if you have fan controller software installed and it is conflicting with settings in the BIOS (however this is sort of unlikely b/c if the software can control the the fans it should in theory be able to override the fan settings in the BIOS)

it revs constantly whenever the pc is turned on so im not sure what to do, but thanks for your input

fitchx has good advice

so try to plug it into a different fan header if you can then restart and see what happens

i seem to have fixed it, i updated the bios on the mobo and got fanxpert from asus so it seems to be all good now thanks everyone

Glad you got it solved!