These alternators have a free-wheeling pulley which often ceases and eventually falls apart. With the belt off you need to check if the pulley will spin in one direction without the alternator turning with it - if not it is ceased and will need replacing.
As already said, it looks like the free wheeling alternator pulley is knackered. what i would do is take the alternator drive belt off and start the engine, if the noise is gone then it is something driven by that belt. then with the belt off, and engine off, grab the alternator pulley and snatch it in one direction and it should spin (freewheel) and then do it in other direction and it shouldn't freewheel.