I just bought an 08 f150 5.4 4x4 with 3.55 gears with 89k miles on it. I just took it back to the dealership to have the IWEs and such replaced since it was under warranty. But while im driving in 2wd down the road it has been making a low pitch humming noise which is coming from the front end. You can only hear it when windows are up from inside the cab so its not loud, and when its happening you can feel a slight vibration from the footwell. it only really happens between about 30 and 50 mph, and seems slightly louder when decelerating. its not the brakes or a wheel bearing since i know what those sound like. Can anyone help? This thing is really starting to stress me out...

 

You need to inspect the whole front end it could be anything..just because it doesnt sound like what you are used too or have heard before doesnt rule out somehing simple like say a collapsed brake line or something of the sort. Might just be some gear noise. Hard to tell without looking at it. Does it do alright in 4wd?

 

As the front diff is not turning while in 2wd, I think you need to look elsewhere.
On 04 up, the front diff and axles are disconnected from the wheels by the IWE, unlike 97-03. If the front axles are still turning in 2wd (engine running), then you still have problems with the IWE, vacuum hoses, or solenoid.

 

Yeah i wasnt sure if the front still turned on those years but i agree with kingfish

 

I am positive that my front diff spins while in 2wd. I have an '07. I had yukon gears put in and they make a tiny amount of gear noise upon deceleration at high speeds, 55+.

 

Then you have a problem - your IWE's are not unlocking.

 

The front axles/diff/driveshaft should never spin with engine running and in 2wd. Might want to verify that that is true and if it is turning, then take it back to be fixed.

 

They unlock, i put a gopro under there about a month ago when I was offroading and you can clearly see the difference between engaged and not. Maybe it is the rear that I am hearing then. Can't really tell from in the cab I suppose. But it really does sound like the front.