Fan noise under hood 2001 silverado 5.3
 

I have a random noise from under the hood. Sounds like engine is over-reving , like a diesel truck, but it's not,only 1600 rpm. It only does it after driving about 60 miles, if it does it at all, and never does it when i'm under the hood, so I can't tell where it's coming from. Have a long trip ahead, so i want to fix it. Anyone ever heard of anything like this?
Thanks

Clutch fan?.....They are supposed to make noise. Once heated up, they lock to move more air. Maybe that is what you are hearing

Will check the clutch, but this noise is new, have driven it 213,000 miles.
Thanks

The fan should be able to freewheel a little when the engine is off. It should have only a little drag when you spin it, if it's real hard to spin, then chances are the fan clutch has failed.