If it is happening with the climate control off, it could be the temp. sensor in the head going bad. This is a brass colored sensor with green plastic. It will have one "2" wire connector and one "1" wire connector. Will be just past about half way back on the head new the air cleaner. When you start the engine and the fan comes on, pull the "2" wire connector off and see if the fan stops. If it does, there is a real good chance the sensor is failing. Seems to me the sensor price was bearable, but was a pain to install since you had to drain half the coolant to avoid leakage, and then had to remove the valve cover in order to get a wrench on the sensor. See if the a/c clutch will engage when the fan is on. If not, then check the sensor.