Q: Grinding noise while driving 2013 Ford F150

Hearing a grinding noise while driving. Took it to the Ford Dealership in Fort Collins, CO. They replaced a solenoid? in the 4-wheel drive system saying that the 4-wheel drive was trying to engage - thus the grinding noise. Two months have passed - noise is back again. Back to the same dealership. Now they are saying that they don't know what it is and they will not do additional tests/diagnosis of the 4-wheel drive system. When I drive slowly and switch into 4-wheel drive, I hear the same grinding noise! Getting frustrated. Any thoughts/advice. They did also determine that the rotors on the breaks are warped - would that be causing the noise? I do feel a vibration when braking. . . . but still have the noise. Noise is really noticeable around 20 mph and 10 mph - when not stepping on the brakes.

My car has 52000 miles.
My car has an automatic transmission.