Last weekend my radiator fan started coming on (very load and fast rpm) each time I start my car (within a few seconds of startup). The fan continues to run the entire time. The same thing happens when the car is cold or hot (recently driven). I swear it sounds like it is going to take off. Any suggestions on what is causing this? Thanks!

2007 4.8liter X5
140,000 miles
the outside temperature has been in the 40s F.