2003 Ford Expedition Noise, Flashing 4H and 4L lights

Dealer has not been able to figure this out yet. So far, replaced 4WD front wheel actuators, vac. Resv, vac. Lines, solinoid, and ball joints.

In 2 wheel drive mode, I have an occasional grinding noise and more often a loud thump when accelerating from a stop and, now, even while driving. The noise comes from, most often, the front driver side wheel. The noise sounds like a rachet is spinning, spinning, spinning, then heavy thump, as if something catches (no handling difference or hesitation in car. Only rachet noise followed by thump.) The noise is identical to the noise one can produce when shifting from 4H to 4L (4L activated when 4L is selected and then car is put in neutral. Thump is 4WD gears shifting).

Typically, but not always, both the 4H and 4L dashboard lights flash, at the same time, just before the thump noises start. However, the lights can flash with no thump and the thump can happen without any lights flashing. There are no codes resulting from the flashing lights.

So far, extended warranty has taken care of the over $1000 bills to repair. But with the problem not repaired, the warranty co will start balking at the fix thinking that the dealer is just throwing darts at a dart board. Any ideas.