Where in the engine compartment is this exactly?
And did it fix your problem?

 

Sounds like a bad Fan Clutch to me might need a new one.

 

I was just gonna suggest that, especially since he has an upgraded flex fan,

Mabey the clutch isnt going bad but rather just turning on under load on hills from heat.. I once had a problem with one of those flex fans, when the clutch turns on those things REALLY move alot of air, I had one suck itself right into a radiator at the drag strip in an S10.

 

The fan kit I have doesn't have a clutch, they are dual electric fans. I thought my loss in power was the fan clutch so I did away with the fan running off the motor all together.

http://www.flex-a-lite.com/auto/html/27inch-electric.html

These are the fans I have. #295

 

how about a broken pipe between the egr valve and the exhaust manifold, so when the egr opens, it sucks air in instead of exhaust???

 

Hmmm, another good hint.

I'll look into it.
Thanks!

 

I have a 1988 F250 4x4 351 and believe that i have heard the same noise that Fallguy is telling about. I hear this noise when accelerating up an incline or on the open highway. I had though that is sounded more like the fuel injection was pushing more fuel as I can manipulate the noise by feathering the gas, back off and it stops, push the pedal and can still hear till less power is needed.
Just my tidbit if info.

 

Anyone considered an exhaust leak? I know mine make s alot of sound from the passenger sid eheader beaing loose.

 

I thought about that..... but only under load? and it comes and go's?

I'm really stumped, I'm gonna check the Fuel Pressure Regulator. And take a look for loose or damaged vacuum lines with a can of nitro clean I can get from work.(like brake clean but doesn't eat up plastic parts)