Had a similar problem on a passat-the alternator pulley had broken up and parted company with the rotor shaft.A new pulley can be bought from VW (about £35)but you will need the special tool to lock the alternator shaft (splined) while the pulley is undone.(snap-on do it).BTW the same problem affects the Galaxy/sharan models where the polyvee belt flaps when the pas is loaded up when the car is stationary.