Transfer Case Noise
There are different types of transfer cases. Many part-time 4WD vehicles contain transfer cases that have electric motors and solenoids to control case operation. The control module sends a signal to a motor that moves set of rails and forks up or down according to the driver's commands.
The range or Hi Low fork in this transfer case moves a sleeve that controls the planetary carrier. By moving the sleeve from the sun gear (1:1) to the planetary carrier, torque is increased over speed with a gear reduction of around (2.75:1). This low gear ratio varies between manufacturers. The mode or front drive fork moves a synchronizer sleeve that locks the drive sprocket to the output shaft transferring power to the driven sprocket through a thick chain; this engages 4WD. Many vehicles use an open center differential transfer case that contains an additional planetary gearset to compensate for the front and rear axles speed differential.
Chain driven cases are sometimes controlled by vacuum. Make sure the transfer case receives proper engine vacuum to insure proper shifting. This is usually around 18-21 "hg depending on factors such as elevation. It's vital to understand the type of transfer case before beginning any diagnostics.