This is normal behavior on the Late 2005 models. My 2.0 DC does the same thing, although I've never noticed the fans going slower, but maybe that's just my negligence.
As far as I'm aware, that's just another one of the differences between the Late 2005 PCIe models and every preceding version before it, like how they lack the 'whoom' at power on, and instead just growl. And like how the fans no longer go into high gear every time an application is opened, etc.
Mostly, the fans were a lot more lively on the older ones, and to be fair, I think a real sense of "POWER" was lost when the fans went mute compared to when they were still loudmouths...