2006 voltmeter fluctuation

Ok everyone I have a 2006 sierra 5.3. This has happen twice to me now. This morning it was probably around -5 degrees out started my truck with remote start and it started fine. After running for 15 mins I got in it to go to work. When went to pull out of the driveway I noticed when hitting the gas my headlights getting brighter and then dimmer when not hitting the gas. When I looked at voltmeter gauge and when not pressing the accelerator it was about 12. When hitting accelerator it would go up to 14. It did this for about 15 mins or so stopping and going. On the way home today it didn't fluctuate. When I got home I tested the alternator with volt meter. It was reading 15.3 volts. From what I read on the internet it says the fluctuation is normal and so is the 15.3 volts. I read 2005 trucks and up have regulated voltage control. Even talks about the gauge fluctuating in the owners manual. I had the battery and alternator tested both tested good. Alternator does have a little noise in it when spinning the pulley by hand. I'm just not convinced that 15.3 volts is normal. The battery isn't original it had been replaced by the previous owner. So what do you all think? Is my alternator possibly going out. Any help is appreciated

These trucks have a 'smart charging system' type of setup, so they allow for alot of variation involving the voltage. The thought process being that the battery starts the truck, then the alternator takes over when running and the smart charging regulates when to slow down the alternator to not overcharge, and conversely charge when system components are being asked for usage. Although these are different systems, I tend to go the route of basics first. Unhook and clean all the battery cable connections top to bottom, grounds too, to make sure you have a good clean connection. Then load test that battery KOEO(key on engine off). Then test the alt KOEO. Then start the truck and test the running voltage of the battery. Then also do the alt KOER(key on engine running) test once again. Along the way, you might find something. If all remains the same and the alt reads 15.3 still or higher, I'd suspect that. If battery is original or old, consider this also. Batteries get maybe 3, 4, or 5 years, some more. Depends on type, kind. Just get back to basics and I'm sure the problem will show itself. Good luck.