Hi, and a Warm Welcome to Apple Discussions and the Power Mac G5 Forum!
Well - you don't say which G5 you have - there's a list here
http://support.apple.com/specs/powermac/
Assuming it's not a "Late 2005" dual-core G5
http://support.apple.com/specs/powermac/PowerMac_G5_Late2005.html
which has several LEDs on the main logic board - explained here
http://docs.info.apple.com/article.html?artnum=302527
you could try a test: when the G5 is running normally - i.e. no roaring fan and LED - take off the aluminium side panel (should not affect the fans) and then pull the clear plastic deflector panel outwards from the top with the handle, and then tilt it forwards so that rear-most locating peg on the bottom edge of the deflector panel just clears the slot in which it normally sits.
If you then get
exactly the symptoms which are the problem - red LED on the front (left as you look at from the side) edge of the main logic board (just behind the power switch and front external sockets), and the roaring fan (it's actually just the Drive Bay fan that goes into hyperdrive - as a warning) - then it's possible the silver/white tab on the rear-most locating peg of the clear plastic deflector panel is damaged or defective. Some diagrams here
http://www.info.apple.com/usen/cip/pdf/g5/073-0891.pdf
If the label is damaged you can substitute a piece of shiny gift wrapping tape, as explained here
http://discussions.apple.com/thread.jspa?messageID=2658261�
which also mentions using a torch/flashlight to check whether the sensor inside the slot itself is ok; more detail, plus cleaning the sensor with a can of compressed air, here
http://discussions.apple.com/thread.jspa?messageID=2996565#2996565
Please post again if you have any questions - or if this isn't the solution. Good Luck.