I got an 87 F250 with a 351w. Ive been noticing that when Im driving around 45-60mph, and let off the gas, the engine produces a very loud humming noise. It almost sounds like a jet engine. It only does that at those speeds too. Anything lower then 45 and you dont hear anything. Ive searched the form and didnt find much. Tried replacing the PCV valve but it didnt help. Any ideas?

 

U- joints, to check it 100% you have to pull the driveshaft and flex the joint.

 

Ok, I had the rear u-joint replaced when the trans got rebuilt. Thanks for the tip.

 

Could be your IAC valve too... try unplugging it and going for a ride, see if it still does it.

 

Not fuel injected. I checked the front U-joints and they seem good? No play in the drive shaft. Any other ideas?