My recently-acquired 92 940T has an UNUSUAL characteristic that my silver 92 940T lacks..... The left front speakers make a tone somewhere around 400 Hz, like the annoying tone for a TV color bar test pattern, but only when the headlights are on. Only happens in the left-front speakers, both door and dash. Headunit is a CR-815.

* It is independent of radio volume. The volume of the noise can change from trip to trip varying from low and annoying to LOUD AND OBNOXIOUS (but stays the same volume the entire trip). I've gotten to where I fade to the rear speakers when I drive at night.
* If only the park lights are on [first position of the switch], it doesn't do it.
* It occurs regardless of the engine running or not (eliminating thoughts of an alternator issue).
* If you fade to the rear speakers, it stops depending on the level of fade to the rear
* Turning the radio off stops the noise.

Given that the radio circuit flow is source->volume ctrl->fader->power amps, it seems logical that it would be in the headunit itself, somewhere between the volume and fader controls (which usually consists of a straight wire!). BUT!!

Illumination is through a single wire, and the problem doesn't kick off the when the park lights are on. How would the headunit know the headlights were on vs. the park lights only? This has got to be some strange reverse-flow oscillation through a common shared ground point that's gone south. Is there a common grounding point the two systems share? Where are the ground tie-ins for each of the systems? Do I have a flakey relay? I've already used large amounts of DeOxit on all the connectors. This one has me stumped!

Thank you!
HA1156W
Dallas TX