After maxing up the volume I observed the noise is heard whenever I do any action like scrolling, maximising or minimizing windows, even just moving the mouse cursor.. but it is not a problem with the mouse because even when I unplug it and use keyboard to do these it is still heard. And I don't have access to tv tuner, AUX, PC-speaker and other options in volume control. I have only those:
On a side notice, when I play those really old DOS games that used PC-speaker the pc speaker sound is heard through the speakers (and is treated like other sound)... first time am seeing something like this...