Whirring noise at constant RPM only.
I first noticed a whirring noise that would appear when cruising. It sounds like a cross between rushing air and two dry metallic surfaces continuously rubbing. The sound appears shortly after coming to a constant speed and "spins up". The sound disappears immediately when in gear and I remove my foot from the throttle ... it will also disappear when I put my foot into the throttle but seems to "catch" before it goes away. I initially suspected it was a transmission noise but I also can reproduce it with the clutch disengaged and the engine revved and held at a constant RPM so I believe that should isolate the source to the engine.
This is a 2000 F-250 w/ the 6-speed manual transmission. Any ideas?