Cooling Fan ran full speed for no reason

iMac G5 17in 1 meg ram less than year old. Was sitting there in sleep mode and fan started running full speed for no reason. Computer screen black and would not wake up with keystroke. Shut down computer via rear panel switch and restart, all fine now. Do not understand why this happened but suspect it is actually a software problem, see below. Anyone have an idea so can understand what happened?
My iMac has the combo drive, not superdrive. I use an external 8x DVD burner for backups and iMovie. Yesterday I burned a iMovie (short) just fine. Then decided to back up my some of my Movie files along with iPhoto and iTunes media files. I tried three times but the burner did not work as usual. The first copy failed to verify and when I put it in the iMac slot, dumb I know, it did not show on the desktop. I restarted the computer to eject the disk, worked ok. The second copy did not work either, but did not put it in the iMac. The third copy burned but took a few hours to get 39% verified. I tried to stop the process with force quit, but it would not quit. Finally shut down the iMac via rear panel power button as a last resort as nothing on the desktop would respond to a mouse click. Computer restarted ok and worked fine for several hours until it went to sleep as I did last night.
I know using the power button is a last resort and possibly there is a keystroke that can force a proper shutdown. Also suspect the DVD external drive is balmy now, BenQ. But was amazed that the iMac still seemingly had some bug up its operating system this morning causing it to freeze and the fan to run full speed.
Again, any ideas...now be nice....I have tender ears
btw have three year warranty and just love the Mac, am going to sell my Dell laptop and get a PowerBook real soon.

Well the problem is fixed. I did call Apple Support and they ran me through the hardware utility. The problem I reported was that the system took forever to startup and that once the fan ran at full speed for no reason. I was not aware of the slow boot when I wrote this question so did not refer to it. But, after the DVD debacle my iMac would hang on the black screen just after the chime for about a minute or longer. The system worked fine once booted. This long boot came from both restart and a cold start. Think it happened once when I was trying to wake it from sleep yesterday, that is when I shut off the 'puter with the poer button. Anyway, the disk utility showed no problems
The Apple Careperson was very nice and stayed with me until the issue was resolved. It turnes out that unplugging my USB hub cured the slow bootup problem, and probably the fan.
So I am guessing that something in the external DVD hardware went and damaged the USB hub. It would be too odd for two devices to fail, on their own, at the same time.
The good news is that I just found a nice LaCie external DVD with firewire 400 support, so guess what I am buying.
As for the USB hub, well without the external DVD I don't need it right now so will wait to replace it.
Thanks for your help and I hope this may help someone else down the line.
Ed

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This question was solved.
View Solution.

Hello mpw101,
Seems like there was some confusion on the products your received and how it was explained. I have forwarded your thread to the appropriate team for review. Someone should be in contact with you shortly so you can discuss your HP customer care experience.
If I have helped you in any way click the Kudos button to say Thanks.
The community works together, click Accept as Solution on the post that solves your issue for other members of the community to benefit from the solution.
- Friendship is magical.

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Get free iStat Pro. This code is obviously related to the HD, but I don't know if it necessarily means the HD is failing. It will show the fan rpms to see which fan is revving up. If it's only the HD fan that is climbing up, this may be related to the HD temperature sensor being faulty. Thing is, the sensor is probably integrated into the drive, so if it's only that that's failing, it will still involve replacing the drive.
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The iMac gets slower the longer I use the Internet - to the point that it takes several seconds for each typed character to appear. This is very noticeable with the online browser game Forge of Empires. But, it happens even if not logged on to that game.
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1st thing : from my point of view upgrade your mac os to latest version.
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2nd thing : then upgrade your all browser plugins to latest version.
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