I am pretty sure the fan is two speed. It goes on high when temps are high and/or the a/c is on. If the fan is just on high it is probably working fine, but you could have a problem with the coolant thermostat or be low on coolant. If it is just making a lot of noise, it could be a bearning in the fan motor going bad. Mine goes on high when sitting in traffic on a hot Texas day or afer running the car hard, I can hear it come on.