It's an "overrun" pulley. It only drives the alternator in the forward direction. It allows the alternator to spin faster than the engine if engine RPMs drop suddenly. They do fail, and they are on all kinds of cars since about 2005. That noise is a possibility, but I haven't seen or heard a failed overrun pulley in person.
E92 335i/6MT Dinan Stage 3: intercooler, oil cooler, intake, flash, exhaust, LW dual-mass flywheel.
Vishnu PWM methanol custom trunk mount, Diffsonline 3-clutch Salisbury custom diff, Ohlins Road and Track coilovers, Vorshlag camber plates, much more...
E28 M5 w/ buncha stuff, R171 SLK350