Has your G750 always made this sound, or is it something new?
If you haven't run any stress tests yet, do me a favor and download:
Prime95
http://www.mersenne.org/freesoft/ (the site looks a little bland, but I promise it is legit)
HWMonitor
http://www.cpuid.com/softwares/hwmonitor.htmlRun a stress test on your CPU with Prime95, monitor temps with HWMonitor and don't let the temp hit 90*C (it shouldn't) and come back and let me know if it triggers the sound.
My best guess at the moment is the sound is coming from the CPU fan motor, or something connected to it, since that is the only moving part at that location of the motherboard. It would also explain why the sound is random since the fan motor only work as hard as needed, if the temps stay low then the fan only work on low rpm. I'm trying to put some stress on your CPU to get the fan up to full speed and force the sound.
If the test runs fine, temps stay low, and there is no sound then we will try something else next.