What rear end do you have? And which model BAP did you get? Big Horn, Laramie...
I drove BAP Big Horn 6.4 with 3.73 and it also had a very loud fan until the truck was at operating temperature. I plan to order one with 4.10 rear once the rear air suspension is available.
3.73, Big Horn.
Yes, its fan noise, and not just until op temp, all the time. Its actually hard to talk over under normal acceleration. Hopefully it's not normal. I'm taking it in soon