Is it coming from the top-right hand corner? (near the power button)
If you go to Titles in iMovie HD and preview the default one, it will probably bring it up consistently.
I sent in my first 17" Powerbook because of this, and they replaced the midplane. Still did it. I then had the whole Powerbook replaced with a new one, and this one does it too. I've now been told that this is normal.
I've only ever seen it occur with iMovie HD when previewing titles though, which is something I don't think I've ever done, other than playing around with it when I first got it. (as it was my first time with iLife '05)
If you have a buzzing / sizzling sound when it's idle, this is also "normal" I've been told. You can stop this by installing the CHUD tools off your restore disc and disabling CPU Nap via system preferences.
CHUD tools resets this setting on restart, so add the attached script to your startup to disable it on logging in, if you have the problem.
I haven't monitored it, but I've not noticed any temperature difference with napping disabled.