I was driving ('97 Explorer) at highway speed when I heard something under the hood very loudly and very rapidly increase in RPM. My first thought was the engine had slipped out of drive, but taking my foot off the accelerator had no effect and the RPM gauge was falling.
When I applied accelerator pressure again, whatever was spinning rapidly returned to normal.
When next I took my foot off the gas pedal, the same thing repeated. I applied gas very slightly and the spinning sound returned to normal.
Sounds as if it's coming from under the hood.
Can anyone propose what this might be? - Mike