If I had to start some place, I'd be looking at the digital communication bus that ships information around between on-board computers. It's square-wave information, and square waves are the sum of a sine wave and an infinite number of odd harmonics. Remember, Bob?

I suspect you're hearing some of those harmonics at the "word rate".

Data cables are ideally twisted, shielded pair, and there's nothing in the wiring diagrams to indicate that they are. I haven't torn into any of my harnesses to see for myself, however.

Although less-suspect, another source of square waves that is not engine speed dependent is the pulse-width-modulated panel lights dimmer circuit. Remember, that circuit feeds directly into the radio. However, it should be easy to determine if it's the source. Any noise with the lights off?

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