Mine does it bad too. I've done lots of research on it and the most popular answer I've seen is the alternator bearings. Most guys seem to just get it replaced under warranty. I've heard a couple guys having the alternator refurbished at an alternator shop as well.

 

Let us know what happens. If that fixes the problem, Ill be sure to do the same.
Thanks!

 

After my dealer threw up his arms (under warranty), I finally ended up replacing my alternator (our of warranty) and quiet finally retunred to the engine by on my 2005. When I told them about the fix, they said they'd never seen it cure the problem. Well, it worked perfectly for me!

BTW, my symptons were exactly the same as yours. I described the sound as being like "what you get when you run a wet finger around the top edge of a wine glass." As with you, it was within a very narrow RPM band too.