A little update to my CB noise investigation. I found the radio "noise blanker" settings were suppressing some of the noise. Once disabled, the noise appeared to be plain engine ignition noise. A RPM related buzz. I pulled all the plug coils, scrapped corrosion and made sure all the coils and connectors were seated and snugged correctly. It helped a lot. But there is still a bit of a RPM related buzz. It doesn't appear to be coming over the 12V. I ran the radio on a separate battery with the engine on and off...no difference. Next up is to check the antenna grounding and coax connections.
Have you tried to put a in-line noise reduction ground loop on the power wire to the CB?
that should take care of your problem, or at least reduce it significantly.
Can't believe no one has mentioned this yet.