2011 Ford Expedition. 4WD system works, but in 2WD the hubs keep trying to engage when throttle is applied which obviously causes sounds nobody wants to hear. Have replaced the check valve in the vacuum line-no change, even turned it around in case it was backwards, no change. Any ideas?
Once you know that the vacuum supply is correct and if the solenoid is operating correctly, then you need to prove whether or not the system has any leaks between the solenoid and the hubs. A hand vacuum pump works well for this, and you should be able to apply vacuum to the hubs, (including the lines to the hubs) get them to disengage and hold vacuum.
One last thing to check for is water in the vacuum lines. That can happen when there are leaks down in the hub assemblies.