Just as a follow up to this--I didn't realize this at first, since the truck was making so many other noises, but I eventually realized that it was making the noise even while standing there idling. It goes away when the clutch is depressed and starts up again when you let the clutch pedal out.
It's currently up at my mechanic's shop getting the brakes fixed. He has driven it and listened to the noise. His opinion is that the input shaft on the transmission probably has a chipped gear on it. He also suggested that for as little as I will be using the truck that I could probably just run it as is. Not sure I want to do that, as the noise drives me crazy! Plus, I don't want to risk doing any further damage. So at some point I will probably have to deal with it. I may have a lead on a different transmission or two and may eventually end up replacing or repairing it.
Anyhow, just wanted to update.
