Front End

it could be an axle that is making noise. if the boots are ripped and the grease is gone, the joint is not lubricated and will fail. get it checked asap as winter is upon us. Roy

If this is only happening when you are in 4 wheel, then it is most likely due to a vacuum leak on the 4 wheel system. You need to verify that the front wheels are truly being locked in when shifting into 4 wheel. The easiest way is to jack up the vehicle so the tires are not on the ground and put it in gear. Does at least one front wheel turn? If it is not locking in there is a solenoid valve under the battery that runs on vacuum. Look for leaks in this vacuum line. They are noted for developing leaks and then the system will not engage properly. What you are hearing is the splines rubbing/grinding as they are not really engaged, only rubbing on each other.