Freaky fan rev up?

I came back to my Mac Pro Workstation just as it was about to sleep. The screen had gone black with "no signal" text still on it. I hit a key to awaken it and it came back up (I had left Safari open). The next thing that happened was scary. A fan or something in the unit revved up super high and wouldn't stop. I let it run like that for about 30-40 seconds and everything mouse and Safari-wise was fine. I could even click and open my drives during that time. I thought it best to shut it down. It restarted just fine and everything seems hunky dory. What gives?

BTW, is a 2 x 2.66 Dual-Core with 4GB RAM. I'm running OS 10.4.11 Thanks!

Go into System Preferences > Energy Saver > Options and make sure Processor Performance is set to Automatic or Reduced. Machine may have been confused into thinking it was in Single User Mode and fans race. If any more trouble download a little utility called Applejack, print, read the instructions, and run it.