Weird noise from transfer case/shifter...

We have a 2001 4x4 Tracker with a 4 cyl./automatic. Recently while driving there will be a "worbling" noise coming from the transfer case shifter (it's hard to describe, almost like a throaty/deep bird call). Sometimes it will stop by itself, but most times you have to either shift the transfer case to 4WD then back to 2WD, or hold your hand on the shifter to apply some pressure.

Anyone have a similar problem that may help me lead to a diagnosis? Thanks!