[Rembert te Loo]

To add for those also searching for a solution for this,

I had the morning squeal, and it would be louder when I turned my lights on. And be gone some 20 seconds later.
I also determined it must be the belt squealing due to the alternator working harder after starting to recharge the battery.
With a hot engine, or better said not to long after stopping the engine, there was no squeal, since the battery was probably not drained that much yet. (No parasitic discharge, and shorter starting). Nothing to do with the engine temperature being hot.

After some more logical thinking, I finally inspected and found the issue to be a corroded earth strap between engine and chassis/body. So the alternator was always working harder to overcome the voltage drop to keep the correct charging voltage at the battery. One clue leading to me finding this was that the headlights would be dimmer at idle, and normal at 2000 rpm.
The corrosion wasn’t bad enough, that the alternator could not recharge whilst driving, but it would have not lasted much longer. The strap had probably a voltage drop of several Volts.

Checking the squeal, not the sepentine belt, but hard working alternator due to corroded ground straps.

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