A chirp or knock that follows engine speed. A pulley bearing on its way out.
A failing idler or tensioner pulley bearing.
Spin each pulley with the belt off. The rough one is your noise. Cheap part, easy fix.
Reading about sounds only goes so far. The tuner on our home page listens in your browser and tells you what it hears. The app goes further: it names the exact fault, checks it against your vehicle, and tells you how urgent it is.