I've now figured out that the 12V/5V converter (switch in middle of hardwire kit) is the culprit. I tried different battery voltage settings with no change. I then wrapped it a few times in aluminum foil and the problem is now gone. Since mine is vertically oriented I left it open on the ends so there is still some air flow if it generates any heat.
I'd call this an ugly fix but it's working for now. Will wait and see what the Viofo tech guys come back with. I'm guessing they have a fair amount of variance in the RF noise in their builds.
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