I've been struggling with constant grinding noise on my truck. I've taken 5 trips to two different dealerships because the truck is still under the powertrain warranty. They can't figure it out, and its driving me nuts.

Description of the noise
1. The noise starts when I exceed 20mph but continues until I stop
2. It sounds like a spinning metal on metal noise coming from the driver's side wheel well area which speeds up and slows down with the wheel
3. It does not change when I turn or go over bumps
4. It does not change when I apply the brakes
5. Dealerships have replaced the IWE and actuator (2x) - no change in the noise
6. I am getting very poor gas mileage. Around 11mpg city when the truck used to be pretty consistent around 15mpg city.
7. The grinding metal noise is unchanged when I shift the vehicle into 4hi while driving
8. Noise not audible with the windows up, but is very apparent when I'm driving along next to a wall or parked cars or other cars in traffic

The problem first presented itself after my wife drove approximately 3 hours on the highway. I noticed that the axle shaft was stuck "engaged" even when in 2wd, I could feel it in the steering. Replacing the IWE and actuator has fixed this problem. However, that noise is still there. I'm wondering if it was possible that something was damaged from the axle shaft being engaged as my wife drove along at 80mph for 3 hours.

Here's a video:
I was hoping to get this fixed with a warranty claim, but, at this point, I'm ready to just deal with it myself.

Any ideas? I'm leaning towards a bearing, since the noise does not change when I engage 4wd. But, typically a bearing will sound different when turning and the original issue was with the vacuum 4x4 system, so I'm a little hesitant there. Also, unless its a coincidence, a bearing wouldn't affect milage as much as this.

A vacuum leak could affect both the 4x4 system and the mileage (right?) - just spit balling

2014 F-150 xlt 4x4 w/ ecoboost and 33k miles

 

If it was the IWE, the sound would go away when in 4h as they should be engaged. Especially, after being replaced (assuming they were installed correctly).

If it really is part of the vacuum system for the IWE, I'd check the check valve, solenoid and for partially plugged/cracked vacuum lines. Also verify line connections are tight and not cracked. Especially down the wheels and frame. Move that line a little as a couple members had that fitting be fine going straight but, turning would open the crack.

Another member had a similar sound that turned out to be the front diff.

I've also read the sound could be a loose bracket above one or both catalytic converters.

Even something stuck on top of one of the heat shields (stick, rock, etcetera).

Just tossing ideas out there. I only heard the sound once or twice in your video so I couldn't get a good idea of the sound.