Users are beginning to report excessive fan noise emanating from Apple's Late-2008 MacBooks and MacBook Pros. One MacFixIt reader writes:
"The first day with my 2.53 GHz unibody macbook pro all was silent, after a day the top left fan is now making a lot more noise all the time and can clearly be heard. I have read on macrumors forum and apple forums of a few people with similar issues."
Excessive fan noise can be caused by spiking processor usage and various other factors. See this tutorial for more information on killing hung processes cthat can inordinately spike processor usage. In some cases, simply killing a process and allowing it to restart will alleviate the issue.
Apple issued SMC updates for the previous generation of MacBooks that dramatically reduced excessive fan noise.
Though not a viable long-term solution, some users have found that turning off one of the MacBook Pro's processor cores eliminates that noise.
In order to turn off one processor, you need to install Apple's Developer Tools (from the install discs included with your MacBook). You will then have access to a System Preference pane labeled "Processor" that can toggle between single and dual-core operation.