Bearings in one of the pulleys is bad.
Thats one possible explanation. It could probably be confirmed if the pitch becomes a bit higher if while in park, and you rev the engine just a tad bit.
A high pitch sound can be very confusing on its location if you turn your head left and right. (doppler effect)
Another possibility, when my alternator started to go, (about one month prior) it started to make a high pitch sound, although that sounded more like a whistle. I pulled up to the dealer en route to my house, and asked the tech to listen, he replied "go inside and set up an appointment so we can diagnose better".
Although that was the correct answer for him to say, he really boiled my blood a bit because I knew it was the alternator, but just wanted a confirmation.
I ended up needing that appointment when my battery light came on a few weeks later.