I posted couple weeks ago that I get a humming noise from 68-70MPH. Outside of that range its quiet. It sounds like pre load in the rear end because it only happens when accelerating. I can let off the go pedal and it will almost go away. I can set my cruise at 70 and on flat road its almost gone. Only shows up when you apply throttle. Changed rear diff fluid ( Was in a hurry and did not pull cover to inspect gears) didn't change the sound and also wasnt any worse either. Took to dealer and went on a ride with tech. I drove. Tech sat in front passenger and all over the rear seat and could not hear anything. I had a friend of mine drive it and I also rode in those places and could NOT hear anything. ONLY time you hear it is in drivers seat. I'm wondering if it could be something other than the diff making the noise???? WHat else can I check??? I checked fluid level in front diff and thats about it. Any other places to check??? Dealer tech ( WHo is actually very good) told me to keep driving it until it got worse. FORD wont address it until you can hear it over the radio LOL . Let me know if you have any other ideas of things I can check. Thanks!

Truck is 3 months old with 4600 miles if that makes a difference.

 

Had the problem in F150..Check your drive shaft/transmission splicing..Also check your engage 4wheel/on off..Good luck

 

Given your general description the noise could be any rotating part from the transmission output shaft to the rear tires....including anything between them.

Odd that it is only heard when you're sitting in the driver's seat.

Some tests to try:

Does the sound change when brakes are applied?

Does the sound change in 4 wheel drive?

Did your friend also hear the noise when he was driving?

Does the noise change when the driver window is lowered?

Does folding in the mirrors change the noise?

Does opening the rear cab sliding window change the noise?

 

OK, some of these things I have checked.
As for applying the brakes, the sound will most certainly stop because if I'm going 68-70 and I hit the brakes to slow down my truck will drop below 68 pretty quickly and it will go quiet.
I live in southern CA and the only time I have ever used 4x4 was to go about 50 feet in 4H up my slippery driveway at about 20MPH. LOL I would have no way of getting up to 68-70 in 4x4.
Funny enough, My friend did NOT hear the noise while driving, so if he owned the truck, it would not be an issue what so ever. lol I bet he heard it, just wasnt a noise that he thought was concerning.
I tried with mirrors in and out, no change. Thought same thing, that it was a harmonic off the mirrors.
I tried with window down and I cannot hear it, wind noise totally covers up the sound.
I have not tried it with rear sliding window open.

Tech said with rear diffs you can usually hear them really good from back seat. Thats where he rode first.

Thanks for everything guys! I don't think its something thats going to let loose and be catastrophic it just bothers me LOL . I had a dodge that made a similar noise from 60-62 fresh from the factory and just needed a new gear set.

Forgot to mention the truck is brand new with 4600 miles if that makes a difference

 

I do have a regular service appointment next week and was going to have them check the ujoints as well.

 

Try driving without your radio antenna. Could also try folding in the mirrors on one test. If you have a bed cover try running with it in different positions.