94 GT 5.0HO Idle Noise

Could also be the tensioner clicking when it bottoms out, if the belt's too long. From the video I agree with the alternator theory, but it's hard to tell where the noise is originating in 2D. I'd suggest a length of heater-hose held up to your ear and move it around the accessories (don't get it caught in the belt) until you pinpoint the location. You're guaranteed to look stupid while doing it, but it works very well.

It almost sounds like electrical arcing inside the alternator.