>The noise is low-frequency and it reverberates throughout the bus. I can't
>tell where it's coming from, it depends which way I turn my head.  The
>noise begins precisely at 43 miles per hour, and increases in frequency and
>volume the faster the bus goes. It doesn't grind; it's somewhere between a
>hum and a sustained moan. There is a vibration to it but the vibration is
>tactile, not aural; you can only feel it.
>
>The noise disappears entirely when the bus is taking curves. It only
>happens on straights.

The curves bit leads me to believe wheel bearings (check both inner 
and outer) or cv joints.

>The noise does not care a bit about engine speed. Doesn't matter whether
>I'm in fourth or third. But I can make it cycle with the accelerator,
>putting it under load and backing off. It's much louder under load.

This also makes me suspect cv joints.

Another possibility is harmonic resonance.  In my '77 Westy I was 
sure my tranny had started moaning.  It also started getting hard to 
shift down into 1st and shifting in general was getting sloppy.  I 
replaced the front nylon shift bushing (the old one had worn apart 
and finally fell out!) and my shifting problem was solved.  My tranny 
"moaning" also disappeared.  Turns out that without the nylon bushing 
there, the metal against metal was just transfering vibration and 
causing the noise (and yes, it was *loud*).

>The day before the noise came to town, I had adjusted all the brake shoes
>and the parking brake. The cables are old and slack and I could not
>possibly have locked the parking brake on even if I didn't know what I was
>doing. None of the shoes are locked on. None of the hubs are hot when I
>stop. Transmission isn't hot, either.

This MAY be your problem.  Mere coincidence?  Maybe, maybe not.  You 
said the hubs were not hot, but how about the perimeter of the drums?  
Did you bleed the lines?  How well did the fluid flow?  Was it clear 
or dark & muddy?

When I do brake adjustments, I back off both stars until wheel spins 
freely.  I then adjust the star until the wheel is locked into place.  
I then back the star off one click at a time until the wheel makes 
one rotation when spun by hand.  Then I adjust the other star until 
the wheel locks again.  Back off one click at a time again until 
wheel makes one rotation again.  (There may be a little additional 
brake pad wear from this method but I have never gotten bad final 
results).

Do this for all 4 wheels.  Then bleed the system (it's important to 
adjust BEFORE bleeding).  Bleed from furthest to closest to master 
cylinder (RR, LR, RF, LF).  Bleed until fluid coming out looks as 
good as fluid going in (may take 2 or 3 times going through the 
bleeding procedure).

Take a test drive to "heat up" the system.  Check each wheel again.
Do any of them bind before one full rotation?  If so, either the 
brake return springs have lost their strength or the rubber brake 
hoses have become restricted (most likely the latter).  If either 
of these happens, your master cylinder has enough pressure to push 
brake fluid to the wheel cylinders, but the return springs can't 
push the fluid back to the master cylinder fast enough.

On my "keeper" buses I always replace all the rubber hoses (about 
$40 for 4 hoses).  Cut an old one open and you will see why.  They 
look fine outside, but they break down inside.

Hope it is something cheap!

Dave