Are all the grounding points on a common ground (which sometimes takes straps between parts of the car since there is a lot of plastic creating isolated elements - especially on Fords). Are you using low-noise sparks? Everything shielded? Got a noise trap (see www.powerwerk.com
) in the battery line? Power line going direct to battery in short, direct path (with fusing). Noise cancelling speaker? Have the standard emergency connectors on the power cable (Anderson Powerpoles). Antenna touching metal ground (not paint)? Personal experience on those points and more with my Ford. Great book on this called the Mobile Handbook by Dave Ingram K4TWJ put out by MFJ and all these things are touched on. It just takes a lot of small things to add up to "quiet" operating. But the goal is worth it. Nothing like talking to Slovakia on the way to work.