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.