hey pro. I know there is no sine wave in a dc signal, but you have to remember that those power panels change the voltage/current (however u view it..) going to the glow plug by means of "frequency modulation". Meaning that the smooth dc signal is broken up into short or long, fast or slow pulses to either increase or decrease the amount of power fed. That is what the whine is. Not from hi voltage as was suggested, but from all sortss of funny frequencies inside the panel as it modulates the dc pulse. The keyword here is obviously frequency and i know that AM TX/RX are a bit more sensitive to this interferance. What you said before about the foil is a great idea but maybe think about screening the actual glow plug lead with it??? I have some EMF screened cable here from work which is just that. Some cable with a foil or braided wire screen around it. But Australia is a long way from the US so maybe the foil will work fine. There should be no real interferance from the actual motor once the glow ignitor is disconnected because there is nothing in it to generrate an EMF (electro magnetic field). The plug is hot from the igniting fuel, not any electricity so we can rule that out. Try the foil thing or maybe chaging the crystal u are using. It may just be at a very similar frequency to the glow ignitor. I hope this is helpful or at least informative.
rev.