On occasion, sometimes daily, while driving there will be a somewhat high-pitched harmonic (I guess that's the right term) tone that becomes quite audible. To me it does not sound like it's coming from a mechanical source, rather more electronic. It sounds almost the same as when you take a drinking glass, wet your finger and rub it around the top in circles until it emits a tone. It is a lot lower frequency than the turbo charger, plus the turbo sounds perfectly normal during this time. Sometimes it's louder than others, but it always seems to be speed-dependent, not RPM dependent, occuring over a range of 1-3 MPH. The speed at which it occurs does not appear to be consistent from one time to the next, but frequently happens in the approximate 40-45 MPH range. The pitch also does not change as I accelerate through the affected range. After a few minutes, it typically goes away and you cannot hear it at all.
Today it was the loudest it's ever been, occuring at 2 different speed ranges for the first time that I've ever noticed. It had a lower-pitched tone somewhere around 20 or 30 MPH, then when I got out on the highway it had a higher-pitched tone at 59-61 MPH. If I let off the accelerator it would fade away as the speedometer dropped below 59 but would come back fairly loudly as I speeded up and got back into the 59-61 MPH range. After 2 or 3 minutes, it went away as happens quite normally. Also, for the first time today when it was doing it at the 59-61 range, it varied between two different pitches.
This is a very odd thing that I've never heard in another vehicle. I have never been able to pinpoint where it comes from; rather, it sounds like it just sort of fills the cab of the truck. Does anyone have an explanation for me?

Bump the starter with the keyswitch then quickly release to see if it's the lift pump making the noise.
While we're at it, what's your lift pump fuel pressure?

You mean bump the starter while driving down the road? Wouldn't that be a bit bad for the starter and flywheel? I don't yet have a fuel pressure guage so I can't give you an answer to your question. That's one of the next things on my list of things to do to my truck, though.
Thanks for your reply!