My car has always made this slight whistle noise. I wouldn't call it a whistle, but I cannot think of a better term. It sounds like electronic interference, the kind you would hear while trying to tune in an AM/FM dial. It's only audible at idle and slightly above idle. You cannot hear it under the hood, you can only hear it from inside the car. It sounds like its coming from behind the dash, under the steering wheel. It's louder with the heater on. It seems to be directly related to the voltage of the charging system, ie, the noise will change pitch as I activate different accessories such as heater blower, radiator fan, blinkers, reverse lights, headlights, etc. I cannot pinpoint the location of the sound, let alone even guess at what is causing it. The car has no audio system whatsoever, all speakers have been removed. Any ideas?

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It must be the machine that goes "DING" that has gone bananas...
Unless it's a vacuum leak of course..

Its probibly a vacuum leak from the back of your heat and ac control

I will agree here to, so s votes for a vacuum leak.....my 91 Suburban whistles when I change the direction to blow air and when I mess with blower motor speed and radio stuff it whistles more until it seals completely.

Its definitely not that, because my car does that too. This is coming from way up behind the gauge cluster.

Does your car have a "convenience center" or whatever the ding-box is called? It has a speaker and could have gone haywire.. That's what I meant with "the machine that goes DING"...

is it a hissss or static????

Its not a hiss. It has some tone to it. An update, I actually can hear it under the hood. Seems to be coming from the rear of the engine, driver side maybe around the brake booster. It could be a vacuum leak, I'll have to poke around.

When the TBI is set close to it's true minimum air (on mine and one other I've set), the air passing by the blades will cause a 'whilstle' noise.
Mine has a similar sound (though obviously much quieter) to a tea kettle boiling water. If you have an open element, it's much more prominent and sounds as though it's comming from the center vents.
Goodyear highmiler serpentine belts will cause the same noise (among others).
JMO though....

Sounds exactly like a tea kettle. When I get a chance I'll mess around with the IAC calibration (idle screw in the TBI) and see if it changes the sound.