Probably a bad vacuum switch on the transfer case. That switch control the vacuum to the front axle interlock. When they go bad they can stick a bit and let just enough vacuum through to pull the diaphragm for the interlock to a point where the interlock starts to engage and you get a loud buzzing/grinding sound. When you put it into 4X4 the interlock pulls all the way in and the noise stops. To test if this is the problem find the vacuum line that connects to the diaphragm (under the battery tray, vacuum line comes out and across inner fender) Take a pair of vice grips or a tubing clamp and stop any possible vacuum flow. Now see if the noise is gone. If it is the switch is bad. It isn't hard to change but access to it is fun. Jack up and support the vehicle, go under the passenger side and at the top front of the transfer case you will find the switch with a triple vacuum tree attached. The replacement is available in most parts store in the motormite HELP area.
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Thursday, June 20th, 2013 AT 9:47 AM