Check the Alternator belt Tensioner for excessive rattle at idle... have someone rev the engine while you look at the tensioner.
If the tensioner roller moves LESS above idle speeds, then you have a one-way-clutch failure of the alternator.
If you can get to the alternator, you can check the clutch while it is still in the car: (this clip also shows the tensioner vibration / rattle)
Here is what a good clutch / fan tests like in the car (engine off):
This is how to remove the clutch pulley: (Golf TDi as sample).
Sunday, March 10th, 2013 AT 5:47 PM