HighAndDry
do you mean the computer Jim? as in disconnect hard drives etc. or usb cables etc coming from it?
Disconnect all cables (peripherals) that aren't absolutely essential to the machine booting up.
Stripped to the very basics, does the noise persist?
If not, then you know it's one of the peripheral/cables.
I've seen a USB cable cause similar noise. Swapped that out... and the noise was gone.