1999 Chevy Cavalier Battery light coming on but the battery

Today my car started making a bit of whirring noise, it was at a low pitch when driving slow, but as I sped up and changed gears (automatic transimission) the whirring got higher pitched, then it suddendly stopped whirring and a few minutes later my check battery light came on. The battery is new though, the car still drives perfectly fine, its starts and stops, no lights are dim, when I had a friend look at it, he unplugged a wire connected to the altinator then plugged it back in. When I started it the light didnt come on, but halfway driving home it came back on again, please tell me its just one of the relays that needs replacing and not the alinator itself cuz thats a HUGE difference in repair costs!