This truck has 140,000 miles on it.
Yesterday, for the very first time every, I noted that that dash voltage gauge read below 14. It has always read rock-steady just above 14.

I also hear a whine that goes up in pitch with the speed of the engine.
It sounds a lot like an AM radio carrier (like that tone you hear when an
AM radio isn't tuned to any station).

Today, I note that at idle, especially with the A/C on, the voltage on
the dash meter really dips below 14 (like about 2 lines, which is probably
somewhere around 12). The whine is still there.

If I step on the gas and rev the engine up, the meter returns to 14.

The serpentine belt is less than a year old and is good shape.
I do not hear any grinding or slipping noises when I examine under the hood.
The engine starts right up. The starter was replaced a few months ago.

Is this simply the alternator going bad? That's my instinct.

Question 2: How the heck do I figure out WHICH alternator I have. There
is nothing marked on it that I can see other than "Delphi". I have
air conditioning, 4WD, power windows, and a 5.7L V8. But there are
105, 117, 124, and 140 A alternators listed for this stupid thing.
Will the rectifiers and regulator all come with the replacement?

Thank you!

The alternator will be stamped with the rating. It may be hard to see but it will be on there.

The regulator is internal to the alternator so all you need to do is get a new alternator and you should be fine.

Be sure the battery cables are in good shape.

//2000CAYukon

yep. alternator no more.
take it into your local auto parts store, and they should be able to identify it for you if you can't find the numbers.

Look in the glove box for one of these codes. (out of my '98 C/K manual)

KG9=140a
KW2=124a
K60=100a
K68=105a

If you have doubts call your local Chevy dealer and give them your VIN. They will tell you what amp Alt came with the truck.

Just replaced an alt on my friends Tahoe for exactely the same symptoms and it was a good fix.

Also wanted to note your good eyes on the guages.............Steve

None of those 4 codes were in the list of codes in my glove box. But I did read the VIN to a Chevy dealer over the phone, and he identified it as a 100 amp.
I bought a reman replacement (from a parts source I trust implicitly based on past experience, and it's not one of the major chain stores), and dropped it
in Friday night. Works like a charm. Noise gone, voltage just above 14 again at idle. A bit of reading around on the web... it simply looks like one of the diodes shorted, so I was running on 2 out of 3 phases. But frankly, after 140,000 miles, the bearings and brushes in that puppy were old enough that it seemed a much safer play to replace the whole alternator.

Thanks, folks.