My 2011 Fx4 5.0 3.73 E-locker 57k auto spring 2 1/2 lift level 1 1/2 in back 295/60/20 Terra grapplers. Alright while I'm driving when your lightly on the gas pedal I can hear a whistle/ howl it is only when in gear. Noise goes away as soon as I slip it in to neutral and coast with my foot on the gas slightly. Put it back to drive comes right back instantly . When I just let off the pedal and let it coast in gear the noise goes away so it is only when theres a load on it. Can't hear it with windows open. Does not sound like the typical U Joint squeak. Don't think its a pulley on the motor because it doesn't do it in neutral or really change its volume with more Rpms / speed. Any ideas.?

 

Check your air filter and housing, sounds like something in the air system, I had the same problem and found a leaf in my air box ... Drive me crazy for 2 months !!

 

Could be just tranny whining.

 

Does it sound like a fake turbo whistle?
I've had that whistling sound since I bought my truck. It's in the air box somewhere. It's sucking air. I read somewhere that there is a fix for it. But it's never bothered me enough.

Mine does it when I'm just barely pressing the gas pedal when in drive. I can flutter the pedal a little and make it happen.

 

I'll have to check it out to see of I can narrow it down to that

 

Sounding like its coming from more of the rear of the truck

 

my 2013 eboost 3.73 4x4 was doing the same thing. rear pinion carrier bearings were shot. replaced 2x now under warranty the truck now has 32K on it.

 

The rear pinion bearings or the carrier bearing?

 

Rear pinion bearings it is! Its at the local ford dealer getting all fixed up now.... thankfully still under power train warrenty

 

rear pinion bearings.
I actually brought it up a couple times during oil changes and they 'could not replicate the issue' so I kept driving until the radio would almost not go loud enough to cover up the noise.. can you hear it now!! haha

 

Hahaha. When I took it in I made one of their mechanics go for a ride with me so I could make sure they heard the noise