Have a Harbor Freight around you? Invest $8 in a mechanic's stethoscope >
HERE<. You'll pinpoint the source and won't have to remove anything or break out any tools. Removing the belt likely would stop the sound, but there are six accessory pulleys so you'd still have to narrow it down. Also, such noises are often due to the side loads the belt inflicts on the accessory's bearings. With the belt off, you lose the side load so the accessory in question may feel and sound fine.
For you, a stock replacement DR44 alternator is probably fine. I'd recommend an OE one and not a remanufactured as they often have lower quality bearings and/or regulators in them than what originally came in the alternator and lasted hundreds of thousands of miles. I'd take a low-mileage used GM OEM alt over a remanufactured. If you find one for the same price or less, a factory alternator that'd be an upgrade would be part number 13534128 or 13536552. These are 220 amp models and off of '19+ trucks, so most would be decently low mileage. Their prices fluctuate and it looks like they're currently high- mostly around $200 on eBay. If you don't need the full 220 amps (and it sounds like you don't), part number 13536551 is a 170 amp version and is a lot cheaper. I see 'em on eBay as little as $57.
Here's one with 849 miles on it for $84 and free shipping. Oh, these alts are not just an upgrade in amperage output. They're also 6-phase so they put out more amperage a lower RPM, including at idle. So they would charge your battery better than the stock style alt when you're sitting still with the A/C on, which runs the cooling fans and HVC blower motors (considerable power draws).
