Do you notice if the noise occurs all of the time. Does it makes a difference if the AC adapter is plugged in or not?
One thing to try is to boot to safe mode. In Windows 8.1 press the Power button at the Windows login screen or in the Settings charm. Then, press and hold the SHIFT key on your keyboard and click Restart. When Windows prompts you choose safe mode.
Does the notebook still make the noise in safe mode? If so it could possibly be Support Assist at fault.
Go here * Under the "End User" tab, click "SupportAssist for PCs and tablets" * Download/save the file "aulauncher.exe" to your windows desktop * Run the file * Reboot when done * Download/save the hotfix from this share * Double-click the exe file. You should get a message saying “Successfully applied the hotfix, please reboot your system for the changes to take effect”
Please let me know if the sound is present in safe mode or not, and if so did the Support Assist fix make any difference.