I'm sure you've thought of this but just in case, the power supply on the G5's has a sensor built into it where if the clear plastic insert (the large one behind the opening side of the case) it will not function. I played around with mine and got the same fan behavior (sounds like i floored it in my honda) make sure the lower right hand tap (it has a small opaque covering on it) is seating correctly, otherwise the computer will know the insert is gone, and thus, cannot cool itself the way it wants to. it just gets angry until the piece is back in and rebooted. The G5's were built with 4 cooling chambers and this piece keeps them separated. just a thought, hope it helps.