I have a roaring noise that comes and goes in 2 wheel drive only. When shifted to 4x4 the noise stops. A dealer has replace both auto locking hub actuators and the driver side bearing and hub assembly but the problem persists. It is still under powertrain warranty.

 

roaring noise that comes and goes? how long does it go away? May be leaking check valves in the 4 WD system, or the other wheel bearing starting to go bad. Drive it like a drunk driver, side to side and see if the noise changes, if it does, then wheel bearing assy. if not, vacuum leak to front hubs?

 

I may drive twenty or so miles before it starts again. I've swerved back and forth with no effect. It comes and goes driving straight. I thought maybe a vacuum solenoid. ????

 

It sounds like you have a weak vacuum signal to the front hubs. The IWE (4x4) system uses vac to hold open or disengage the front hubs, when the vac goes away the internal spring locks in the hubs. When your vac signal is weak, the hubs try to partially engage causing the noise you hear. When this scenario occurs for a prolonged amount of time permanent damage will occur to the hubs (that's why they were replaced under warranty) and if it is allowed to happen long enough it will prevent the hubs from engaging at all.

Look for a broken or cracked vacuum line or a hole in the line, a bad IWE solenoid or a weak vac signal from the engine. The noise you heard is the symptom, not the problem. Ford has used the same 4x4 system on all 4wd 150's from 2004 to 2014, and I think the new 15's have it too.

 

This thread should help:

https://www.f150online.com/forums/art...then-some.html