Rattly-squeak noise coming from the area of the alternator pulley.
Eventually the rattly-squeak turns into a tortured-metal death-rattly-squeak/squeal, and in my experience, very shortly after this, there is a THUMP followed immediately by the alternator-failure warning lamp (and it all goes quiet again), after which the offending alternator pulley will be found on the top of your belly pan along with the belt!
You can drive the car with the serpentine belt off, as long as the battery will last. There's nothing immediately mission-critical driven by that belt - only the A/C and the alternator. I needed half an hour to get to my buddy's shop ... made it there with plenty of battery charge to spare. I turned off everything possible that was operated by electricity.