Based on your description, I would say the fan clutch is (working correctly) is locking in because of the heat. They are designed to spin the fan at different speeds based on temp. If it's really hot, they will lock and the fan will spin as fast as the waterpump is spinning. As a result, they make a roaring sound. Once they cool down or the outside temps are cooler, there is no need to lock in fully and they don't spin as fast thus not making the noise.
Wednesday, September 2nd, 2009 AT 11:26 AM