Fuzzy,
I can understand why so many posters have focused on the alternator, because the noise is a kind of classic sounding electrical high pitched sound.
However, I suggest that there are other possibilities (I am certainly not ruling out the alt, by the way): could it be a shaft in the drive system ( tranny, drive line, wheel bearing, etc) or the engine/belt driven accessories that has brinnelling. There are several different sounds that worn bearings can make, and I think it is possible that a shaft or a set of worn bearings could make this noise. The noise can be made by very slight, but repetitive imperfections. A water pump, even if gear driven, power steering pump, alternator, a/c compressor and its clutch assembly could produce such a noise.
A transmission that was rebuilt, but had a shaft with a worn surface or a set of bearings that were not replaced, because they looked ok could cause this noise. U-joint needle bearings are also a possibility.
Another possibility is that high amperage surges caused by the alternator/sound system have caused damage to a set of bearings on the truck, which then can create strange noises.
Another unlikely possibility is that at some time tires with a silicon rubber composition (provides longer life)were used on this vehicle, which can generate higher levels of static electricity thereby damaging the wheel bearings (this type of bearing damage has been documented over the years), resulting in the strange noise. There have been many cases of toll operators who have been shocked by static electricity when collecting tolls from drivers of vehicles with silicon compounds in their long life tires. In a previous life, I sometimes replaced electric motor bearings damaged by high levels of static D.C. electrical voltage. The bearing noises sounded a lot like your vehicle. Again, this is a long shot--but my point is that there are many possibilities here.
One last point: bearing clearances can vary by temperature, so it is conceivable that ambient air, lubricant or vehicle temperature or load on the component could change/eliminate the noise. We wish you the best of luck!
Your idea of using the microphone system is a good one.
Larry