I picked up a couple of 32" Vizio E320VA LCD TVs last week and discovered a hum problem when connecting either a ReplayTV or a DVD player using the A/V jacks. As soon as the yellow video cable was plugged in the hum would start. I called Vizio and they had no clue on what to do and scheduled a service call to replace a board. I doubted that would work since the problem was on both units. It wasn't the cable I was using either as I tried several.
I remembered a solution I used 20 years ago when I connected a Toshiba VCR to a Mitsubishi TV - install a Ground Loop Isolator on the audio. Worked like a charm this time too. The GLI, which you can pick up at RS for about $18 or for just 6 bucks from Amazon or ebay is a fast, easy fix. Use the version with two male and two female RCA connectors and use it inline with the existing cable.