While the engine is running and making noise, mist some water on the flat side of the belt. If the noise goes away, the belt is making noise.

When it is making noise, is it a slow chirp, pause, chirp or a constant chirp, chirp? Have a look for the label on the back of the belt and get a sense if the chirp is in time with the label passing across the flat pulleys.

To solve, sand the flat pulleys with some 80 grit to roughen them up a bit.

If the belt label is slick, sand that a bit too. I've had slick labels on new belts give a chirp , pause , chirp. If the belt is old but good condition, sanding the back of the belt a bit helps too.

And now for why you are doing all of this. Micro V belts generally tension and snake around the accessories by using flat pulleys on the back of the belt. These pulleys glaze over / back of belt becomes polished after a while. As the belt is used, it walks back and forth on the flat pulleys a bit causing the rubber to chirp / squeak. By sanding the pulleys they then grip the belt and no more chirp.