To cause a whirring noise there must be something moving. The only things which move in a pc are the fans (in power supply unit, cooling fan on cpu and the fan(s) which draw air through the box) and the disc drives.
If the noise is constant then the only drive to cause concern is the hard drive.
I suggest that you listen carefully to locate where the noise is coming from and this will point you in the right direction to diagnosing the problem. If you feel confident enough to open the box then it may be easier to locate the exact place the noise is coming from, but please be careful as the power must be switched on and you don't want your hair becoming entangled in any of the fans or even worse getting into the mains power inside the power supply unit.