Have you inspected the bearings in your original alternator?
My bearings went out on my Chevy Van at 145K miles even though the regualtor was fine. I replaced the entire unit since I felt I got my $ worth with it lasting 145K miles.
Its easy to overlook the mechanicals on an "electronic" part.
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles