iwe solenoid allow water to enter the vacuum system and you are loosing vacuum on your hubs at random intervals.
Ask the dealer to look up TSB 06-8-15 TSB 06-8-15
more than likely is the culprit. Has similar noise on my 05 4x4. If this is the problem your dealer should have found it before now; its kinda sad on their part. The splines on your hubs are probably shot if its been happening enough. Does it sound like a gear trying to engage? Like trying to jam a manual trans into gear without the clutch?
possible bad iwe solenoid
Intermitent Grinding Noise on 2004 F150 4x4
If this is the cause I would make them pay for it; your brought it to their attention long before your warranty was up and as a result you have more than likely damaged the splines on your hub or more. If you don't have extended warranty.