I have the same issue with my Hitachi HD TV. The noise comes through both the TV and downstream stereo. Apparently, this is caused by the Verizon card, as my other Hitachi uses a DVR and doesn't have this problem. The noise is very loud and at the range where it could cause speaker damage. It almost always happens when local commercials cut in and out.
I called Verizon, and they were useless to the point of rudeness. They bridged a Hitachi tech to the phone call, and were told that this issue was directly related to their card. Then, I was put on hold while the Verizon "techs" were apparently contacted. Their answer was "can't be our card", and that was it. Frankly, I'm inclined to believe Tosihba over Verizon.
The best option is to remove Verizon's card before it damages your TV. If the audio is having problems, then there could be other issues as well. If your system is damaged, I'm sure Verizon won't lift a finger to help you. Would you risk a $2,000 TV over a 5 cent card? Not me.
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