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Ford has a technical service bulletin for uncommanded hub engagement causing a grinding,ratcheting or buzzing type noise. If the vehicle has been having this problem for any length of time then the integrated wheel ends and hubs will need to be inspected for gear shear. I have attached the tsb below.
Did you click on the click here link? It is a pdf file. It involves replacing the iwe solenoid and clearing the vacuum lines. If you have an extended warranty such as a ford esp premium care it should be covered. It would be covered under the factory warranty for 3 years 36k miles only. Tell the dealer the tsb # XXXXX
Some 2004-2006 F-150 and 2006 Mark LT 4X4 vehicles built before 4/24/2006 may exhibit a noise inXXXXXfrom the front Integrated Wheel Ends (IWE) and/or the 4X4 system being inoperative. The IWE solenoid may have ingested water and passed it to the IWE system causing water contamination in the vacuum portion of the system. A shorted IWE solenoid can set the codes C1979 and C1980. If either code is present, the 4WDH light will be inoperative.
Use the following Service Procedure to check and repair the 4X4 system.
Eligible Under Provisions Of New Vehicle Limited Warranty Coverage
NOTE: The information in Technical Service Bulletins is intended for use by trained, professional technicians with the knowledge, tools, and equipment to do the job properly and safely. It informs these technicians of conditions that may occur on some vehicles, or provides information that could assist in proper vehicle service. The procedures should not be performed by "do-it-yourselfers". Do not assume that a condition described affects your car or truck. Contact a Ford, Lincoln, or Mercury dealership to determine whether the Bulletin applies to your vehicle. Warranty Policy and Extended Service Plan documentation determine Warranty and/or Extended Service Plan coverage unless stated otherwise in the TSB article. The information in this Technical Service Bulletin (TSB) was current at the time of printing. Ford Motor Company reserves the right to supercede this information with updates. The most recent information is available through Ford Motor Company's on-line technical resources.