I had the same grinding noise (I think) that you're talking about, and it was indeed the alternator. Almost a growling sound.

I just bought a used alternator on ebay for 100 bucks and installed it myself. Been working great ever since. I went with a used Valeo, becuase I thought there were some incompatibilities with Crossfires that were originally equipped with the Valeo unit and switching to a Bosch unit. Maybe someone can shed some light on this?