I have an Aiwa Receiver (DD 5.1) that makes a noise that sounds like the scratchy noise that you use to balance the speakers. It only happens when I watch HD channels from my cable, and sounds normal when watching regular channels. When trying to watch HD, the "Dolby Digital" flashes on my receiver display. I have my cable box hooked up through a digital audio cable, not the red/white ones. What is this and why is it happening? How do I fix it? Thanks in advance for all help.