My G5 tower is one of the early models. Today I turned it on. Immediately the drive bay fan was running at maximum speed. The Mac's temperature was cold as it had not been used for 12 hours. So the speeding fan could not have been due to the drive bay being overheated. The fan never slows down. Even after a restart it immediately spins at its maximum. Plus it stays at maximum speed all the time. Anyone know what is causing this & how to make it work properly?