Transfer Case Noise?
 

My 2000 GMC Jimmy 4WD with 4 button transfer case makes a clunking or thumping noise occasionally as I am pulling out of a parking space in forward or reverse. The noise will go away within a few seconds of driving and drives normally. I am thinking it has something to do with the vacuum system or engagement of the transfer case. Since it does not happen all the time I do not think it is a CV joint. I normally drive in Auto 4WD. I might try driving in 2WD High for a while to see if it will occur. Would having the transfer case fluid changed help? Any ideas or recommendations would be appreciated. Right now I do not have any space, tools or time to investigate this myself or even change the fluid. I would have to take it in to a shop for any service. Another point is that I have no fault codes at all and the car drives great otherwise.