Alternator noise

I have a re-manufactured Motorcraft alternator in my truck (v6). The alternator has only 20,000 miles on it in three years. About two weeks ago it starting making what I can best describe as a whirling noise and it definitely sounds like it is coming from the alternator and is a bearing going bad. Noise gets louder with higher rpm's and is constant each and every time I drive the truck. I bought a re-manufactured Carquest alternator and that seemed to only make it louder. I then bought a new Carquest alternator which did not fix the noise. (Both from Advanced Auto). Then I replaced the tension-er pulley assembly and then the lower idler pulley. Neither fixed it. Finally I bought a new Duralast battery from AutoZone and that did not work either. Battery tested normal above 12 amps even though it is six years old now. All other pulleys are quiet and have been replaced within the last few years. The belt is two years old and looks good. What is the problem? Belt alignment? It seems fine. The truck has 120,000 miles and is in very good condition. Thanks!