hmm, there’s a difference between the HD spinning, and the fan spinning when trying to cool it. What happens if you put your iBook to sleep (just to be sure, leave the lid open, so it can cool a bit more via the keyboard part)? While I guess sleeping also silences the fans, those may going ‘round a bit longer to cool the last bits.
But never stops: if you leave the iBook alone for an hour, with absolutely no programs running (ie, the login screen after a boot, or your desktop after a boot), it’s still noisy? (keep it in a cool place, outside any sunny spots, just to be sure). That would more likely point to a software problem, since there’s no particular reason the hard drive would be too hot in such a case (although, if it’s a new HD with a new system install, spotlight might be updating it’s database in the mean time). If that’s the case, Apple may not have the appropriate drivers (or at least no optimised ones) for that hard disk.