I think I may have solved the problem. I read that this particular model is not capable of "deep sleep", which is when everything, including fans, drives, etc stop. According to Apple:
While all PowerBook and iBook computers go into "deep sleep" when Sleep
is selected, some older computers do not. Instead, the display turns off
and the hard drive spins down, but many other components (including the
processor) remain on.
Deep sleep is available on these computers:
All PowerBook computers
All iBook computers
Power Mac G4 (AGP Graphics) or later
iMac (Slot Loading) or later
Problem solved. Thanks!