I have a 98 f-150 4.6 that has a terrible roaring sound all the time.When you rev up engine a little it sounds like a plane taking off. I replaced fan clutch and serpentine belt. When serpentine belt is removed and engine running no roaring it sounds great. Does anyone have any idea to what it might be? Thanks.

 

Fan clutch is probably gone. When you first start an engine with a fan clutch, it should roar then quiet down. I suspect that your clutch is no longer functioning.