I should have looked further - there is already an extensive thread on this here:
I bought a series 450 TV (LN32A450C1D) in the spring as a 2nd TV so my wife didn't have to watch baseball every night with me
It's plugged directly into the wall (not through a cable provider box) and on the HD channels the audio will intermittently go from normal to this robotic "vibrating" sound. It sometimes passes after a few seconds and sometimes I have to change the channel and return to make it go away. It's almost as if the audio isn't locking on.
I know that the cables are good and this past weekend I took the TV into the room where my other TV is and I took the cable out of the cable box for that TV and plugged it into this one and it did the same after a few minutes. I did not try going through the cable box because that's not how the TV is being used.
If anyone can help me figure out how to resolve this I would greatly appreciate it. Thanks!