Remove the belt & see if ANY of the belt-driven accessories spin roughly or loose or ???

Alternators should last longer than that, but anything can break at any time. Especially after the warranty ends.

 

This may sound crazy but works to find a noise. You can take a long screwdriver and put the tip on the component you are checking to see if the noise is coming found and put your ear to the handle and you will hear the sound that component is making. Be careful with the engine running to not let the tip of the screwdriver slip off into something turning.

 

Yes, that absolutely works, as long as you don't get stuck in the spinning belt.
The other kind of poor-man's stethoscope is a scrap piece of garden hose.
Depending on the source of the noise, one or the other type of aid will work better.

 

I have found that adding a cone to the end of the long screwdiver works great cuz then you don't have to actually have it against your ear. I use an old washer fluid bottle with the bottom cut off for this. It is a poor man's stethoscope/megaphone.