2011 F-150 XLT 4WD 5.0L V8, 238,000 miles, second owner at 221,000. 9.75 rear diff, no locker.

Unknown previous maintenance history before 221,000. Was a farm truck, according to previous license plate.

Recently had icy, snowy conditions in central Oklahoma. Previously drove another F-150 that utilized 4A as well as 4H, instead of just 4H. Genuinely didn't know the difference as I've always thought as long as I don't have a welded diff or lockers, nothing should really go wrong. Utilized 4H for over 300 combined miles of terrible traction roadways. Not long after (In 2WD), began noticing a slightly grindy hum coming from the rear end when turning, both directions. Began to get worse as weeks went by. Very noticeable when full lock turning at slow speeds, but also noticeable even at around 25-30 MPH if cornering hard enough. Noise not present when driving straight forward or backward.
Called a few shops to see what pricing would be to have them do a clutch pack replacement. Didn't like their answers so took to YouTube and completed the job myself, not too difficult, saved $800.
Note: Absolutely no slop in the ring and pinion.
No obvious gouges where gears mesh. Did not replace carrier bearings. Old clutches did not seem awfully worn, but new ones looked to be in way better shape. (Obviously)
Did not solve mystery noise. Re-opened the rear diff (bye-bye $100 gear oil and modifier) to remove axle shafts and replace wheel bearings and seals. Should have done it while I was already in there, but I'm still learning this stuff. Packed new bearings with bearing grease and completed the installation. Again, mystery noise has not been ridden of. Noise has not worsened or gotten better. IWE 4X4 system operational and I tested the system to verify function. Did not solve issue.

I am not certain if this noise is causing significant damage, but It's sure obnoxious. I recognize this vehicle has quite a lot of mileage, but I like the truck and am willing to put in the effort to fix it.
Additionally, after moving forward with my repairs, I'm wondering if the issue is not rear differential-related. It sure sounds like it, but I'm wondering if the transfer case is damaged, or that the shift-on-the-fly motor isn't fully disengaging when returning to 4X2. Motor doesn't sound sluggish or exaggeratingly clunky. Disconnected harness from motor, cleaned it up with electronics cleaner spray.

Once in a blue moon, I'd say one out of every thirty 4x4 engagements, the "CHECK 4X4" message will appear, alongside the orange wrench icon, before and after I've messed with the wiring.

Any advice or suggestions would be greatly valued. Will provide additional details if requested. Thank you for your time.

 

Your symptoms sound like possible IWE engagement problem. If you engage 4x4 does the noise go away?

 

I lifted the entire truck up and thouroughly tested the IWE hubs, they don't seem to be malfuncitoning and are operating as intended. Tested them by removing the vacuum lines and spinning the wheels. Spun the CV Shafts, no audible sounds.

 

How about pinion bearings?

 

The IWE is locked and unlocked by way of vacuum.

Disconnecting vacuum lines (if engine is off) is not necessary to do the test you have confirmed you've done. At that point, hubs are locked by default (engine off).
You have proven to us and yourself the hubs are locked by witnessing the CV/axle rotate.
Note: You did state you 'fully tested the system'. To confirm, you have checked proper vacuum is being reached and maintained? You confirmed the hub engages/disengages with the presence of vacuum?

Do you experience any binding when you're doing these lock-to-lock tests where you say the noise persist. Curious if one wheel is remaining locked, however I don't know if that would actually cause a bind though.

As for IWE, if all vacuum tests have passed the tests, it is possible the IWE itself is bad internally and not fully disengage. You'd normally experience a rattle type sound at all times in 2wd if the hub is unable to fully unlock. The other symptoms you mention don't seem to be related to a partially engaged hub. "A grindy hum" is sort of an IWE thing. My own opinion would be the IWE and hub are matching face and you're hearing that but I don't know if that is even possible. Generally the IWE won't retract far enough to get off the teeth, so they sit there eating at each other until the teeth round off. That sounds like a ratchet or like you're dragging a metal shovel.

 

Probably not related to your current problem, but for future reference 4A and 4X4 are very different. 4A or AWD sends power to all 4 wheels, but will allow them to turn at different speeds. When you go around a corner each of the 4 wheels takes a different path and needs to turn at different speeds. The inside wheels are much slower than the outside wheels. 4A or AWD is safe to use on any road surface and at any speed. It is a very good option for snow and ice as well as light mud or sand and even improves traction on hard dry pavement. It does use a little more fuel and is a more complex and expensive system to put in vehicles. It is not the best choice for really deep snow, mud or harsher off road conditions.

In 4X4 the transfer case locks the front and rear axle together trying to force all 4 wheels to turn at the same speed. Which is great in deep mud, snow or rougher off road driving. But if you are driving in anything but a straight line it will bind up and potentially break stuff. Unless you're on a road surface with poor traction that will allow the inside wheels to spin slightly as you turn. True 4X4 is a less expensive, old school system that it really intended for harsh off roading. Most people would be better off with AWD, or Ford's 4A system. But it costs more.

If you drove 300 miles on ice and snow covered roads that were completely covered in ice/snow that is fine. But if the road surface is patchy with clear spots and other snow covered spots then you need to be shifting in and out of 4X4 and only use it as needed. With 4A, or anything with AWD, you just drive and don't worry about changing road conditions.

This doesn't sound like a transfer case issue. But using 4X4 in the wrong spots will lead to a transfer case failure. BTDT and paid the bill.

 

Not trying to be rude but static test (lifted) of IWEs does not actually do what I am asking. If you engage 4x4 and drive down the road does the noise go away?

 

I appreciate all input. No, the noise does not go away when driving down the road in 4x4. The noise is only present when there is a difference in wheel speed between the rear wheels. I lifted the rear end and pulled off the driver side wheel, brake caliper and rotor. Passenger side tire was let down to rest on the ground to force 100% of power to go to that specific side. I left the vehicle in 2WD and put it in drive. The passenger side wheel did not move, and the driver side spun rapidly. The absolutely horror of a sound began to emanate from the hub, seemingly. Had a friend assist with this process and I physically touched the hub while it was spinning and it was vibrating intensely. As soon as the passenger side wheel was lifted from the ground and both sides began spinning at the same speed, the noise vanished entirely. This process was recreated using the brakes in the video attached.
IMG_8717.mov

 

Thank you for the info, recently watched a video about it and definitely hope I didn't do much harm. I appreciate your input.

 

I tested the lines by pulling a manual vacuum, pulled it to 25 PSI on both sides from up near the brake booster. Left each side for about 30 minutes and came back to about 24.5 PSI on driver, 24 PSI on passenger. Verified that the check valve is still holding pressure, tempted to just replace it anyway since I messed with it, and the solenoid activates audibly when 4x4 is engaged by the switch.