poorsche1,
If your car has a manual transmission it has an alternator pulley clutch that rotates freely in one direction only. You shouldn't try to rotate it in the other direction because it could cause damage.
A while back I experienced an odd noise that turned out to be my alternator clutch pulley going bad. It happened right after I installed a new water pump and a whole bunch of other parts so I already had my access panel off. It was rather easy to hear that the noise was coming from my alternator.
I would describe the noise differently from your description, but that doesn't mean it isn't the same problem. In my case it was obvious from idle. If you've ever heard an automobile AC compressor cycling on and off frequently due to low freon, my clutch pulley noise was similar.
Here is a thread I started when I experienced my problem.
Best of luck,
ddb
http://www.986forum.com/forums/showthread.php?t=21779