CPU Fan Always Running Full Speed

[Corgiss]

Hi all. I have a Maximus IX code MB that I built a system with a little over 5 years ago. It's ran like a trooper for the entire time. A couple of weeks ago out of no where the radiator fans for my CPU cooler started running full tilt all the time. Up to that point the only time they spun up to max was on initial boot up. The radiator fans are connected to one of the CPU fan headers on the board, not sure which one without looking. Any thoughts or ideas on what might have changed so that the fans do not spin down anymore? Thanks.