Plus two to the guru's suggestion of removing the belt. That will let you know if it's internal to the engine or confirm that it's one of the rotating accessories.

Something in the AC compressor can make exactly the noise you're describing and it travels quite well. That said, if the noise you are hearing through the mechanic's steth can ONLY be heard when it's on the WP and not anywhere else, it's a good bet it is the WP.

Have you slipped off the belt and just checked the WP? Spin it by hand, back and forth, and check for free play, see if it feels/sounds odd.

Good luck.