Sounds like your power supply just isn't up to the task. I don't know how much power consumption goes up when overclocking but the i7 is not exactly sipping power at stock speeds.
Try running Prime again with all unnecessary devices unplugged: optical drive, extra hard drives, sound card removed, etc. If you still have a whine from the power supply, I don't think you're getting 750W from it.
It's also possible that your power supply has multiple 12V rails and your power load is imbalanced. Check the PS label for items like 12V1, 12V2, etc. If you have a multi-rail design, you may to have to figure out how to balance it better.
I didnt consider that, the power supply must be imbalanced cause im running everything even graphics card on the first 12v label. I was wondering why im having oc problems with a 750 watt, never knew i was suppose to balance it. Thanks mate!