Ford Repair: 1998 Ford Explorer Roaring sound, fan clutch, radiator fan

Question
My 1998 explorer has been making a roaring sound for quite a while now. I does not make the roaring sound when idling only when the truck is moving.

Answer
Given the limited scope of information you provided, the only thing I can suggest is to check the radiator fan clutch, it may be locked up. It should spin by hand, with the engine off of course. It can be locked up or coming on too soon, sounds kinda llike an airplane prop???