Battery charging issue, some clues?

[bambbrose]

I've been having one of those lovely intermittent issues the past 3 months.

Here is the rundown:
-Battery has been load tested, and passed without issue (2 years old).
-Alternator has been on-truck tested at parts store and passed easily.

Truck:
2004 GMC 5.3 4x4 SLE Extended Cab 1500


Issue:
Occasionally, upon first start up, I get a "Battery Not Charging" message on the display center. My voltage gauge will show around 11 to 12 volts.

I can hear a very very faint wheeze sound. Not a squeaking belt. Not a grinding bearing, or metal to metal, just a hum. Almost like the hum of an intake sucking in air at idle without the airbox on.

If I drive the truck, after about 1 or 2 minutes of driving, the voltage gauge all of the sudden kicks up the 14.4V immediately, and things are fine from that point onward. It isn't a gradule build up of voltage, but as if the alternator just decided to work out of nowhere. The sound goes away when the voltage kicks up. The sound is so faint you can only hear it with audio off and windows down.

Because it only happens for the brief period in the morning, and at that only once every few weeks, I can't seem to reproduce it for any diagnostics.

I'm worried about it kicking the bucket while on a trip or on a family member, so I'm looking for advice or clues.

The truck has 141k miles, original alternator. Belt looks worn, probably needs replacing sometime soon.

Any ideas on how to check this?