Okay, so it's a rattling noise.
Pop the other belt off, see if it goes away. If it does, it's either the AC gone bad, or possibly a totally shot tensioner (they chatter like that once they go bad)
Look at the tensioner where the alternator is mounted. Is it bucking back and forth?? There's a chance that it could be the S/C and the rotors are hitting each other a little, but I would think that noise would be faster.
Get a long wrench or screwdriver, and put one end against various parts of the engine, and the other end against your ear. It will transmit the sound like a stethoscope. (please dont use the pointy end of anything against your ear)
Do that to the S/C, tensioners, LIM, etc, until you find where it is the loudest. Then, try to isolate exactly where it's coming from.