HD Channels Sound Quality Issue

I have an Insignia 42inch 1080p lcd tv which I bought about a month ago. I don't have a cable box so the cable is directly hooked into tv and i'm using the internal tuner to pick up local hd channels. The picture is excellent on the 8 channels I receive but the sound isn't always great. CBS has a slight distortion every few minutes or so, NBC didn't do it at all til a couple of days ago same with PBS. One of my FOX Channels did it all the time but the other two don't and my ABC channel has never done it. The sound isn't awful but it gets distorted occasionally and it is annoying. I have not noticed any sound problems with any SD channels only the HD and not all the time. Is it my TVs tuner or sound problem when I turn speakers off in tv and go through my surround amp its the same problem so I eliminated the speakers but not the sound processor since I'm going out of the tv do you think a cable box would solve this problem or do you think its a comcast cable problem. I don't know which way to go because so far this tv has worked great and the picture is excellent. Someone suggested maybe a cable splitting problem but wouldn't that make for a bad picture also. Any help would be appreciated before this drives me nuts.