About two months ago, my ASUS F1A75-M PRO starting issuing a quiet high-pitched squeal through the rear sound jacks (I had to turn off my audio system whenever the system went to sleep). The squeal is still there, but now there is no sound signal coming from the rear jacks when the unit is not sleeping. I tried powering off, including AC feed. I am have a tech background, but my PC hardware knowledge is obsolete. Any ideas?