YannoRoss,
To do the hard reset on your laptop you can hold down the power button for 30 sec with the power cord removed.
The problem may be from a bad ground.
To make sure that it is not software that is causing the problem, you can run a refresh of windows.
Here is a link that will explain and walk you through the step of a refresh.
If you run the refresh and your still not able to get rid of the sound.
Here is a link to call HP and see what repair options they may have.
Let me know how everything goes.