Quick thoughts:
- If the alternator really is making most of that noise, it's most likely bearings that are failing within the alternator. Replacing the alternator will fix it. Thankfully, Windstar alternators are reasonably priced, have good access and are easy to replace. Note that my experience is on my '00 3.8 Windstar, but your 3.0 Windstar's alternator appears to be mounted in the same manner.
- Make sure it really is the alternator that's making this noise! One quick method: Remove the serpentine belt and spin the alternator by hand. Do you hear the same kind of noise, and feel harshness in the alternator shaft and bearings? Then it's time to replace the alternator.
- Note that the idler pulley is below the alternator pulley. Failing idler pulley bearings can also make this noise. Again, just spin the pulley by hand with the serpentine belt removed. It should spin smoothly, without excess play. Idler pulleys are pretty cheap and easy to replace.
- Another item: The power steering pump is right next to the alternator. In my experience, Windstar power steering systems tend to be noisy. I replaced my power steering pump - and even the new pump was pretty noisy. I've found that periodically changing the power steering fluid is a good way to quiet the system, and help it run smoothly. I use Mobil 1 synthetic ATF; check your owner's manual for appropriate power steering fluid options for your vehicle.