The pop may be from the slip yoke at the front end of the drive shaft.
Depending on the type of transfer case you have, the splines that connect the driveshaft slip yoke to the output shaft of the transfer case may or may not not be internally lubricated.
In the case that they are not internally lubricated, the splines require grease to be manually applied. This is normally done at driveshaft installation but after many years/miles the grease is dry - or just gone. Regreasing requires dropping the drive shaft and cleaning/greasing the splines.
That is just one common cause, there are any number of others. A GM TSB on driveline clunking:
Drivetrain - Information on Driveline Clunk Noise
INFORMATION
Bulletin No.: 99-04-20-002H
Date: April 11, 2013
Subject: Information on Driveline Clunk Noise
Models:
2014 and Prior GM Passenger Cars and Light Duty Trucks
Supercede:
This bulletin is being revised to add the 2013-2014 model years. Please discard Corporate Bulletin Number 99-04-20-002G (Section 04 - Driveline/Axle).
Some customers of vehicles equipped with automatic transmissions may comment that the vehicle exhibits a clunk noise when shifting between Park and Drive, Park and Reverse, or Drive and Reverse. Similarly, customers of vehicles equipped with automatic or manual transmissions may comment that the vehicle exhibits a clunk noise while driving when the accelerator is quickly depressed and then released. Tipping into the throttle after deceleration can also result in some level of clunk as the driveline is loaded in one direction (coast) then with throttle reapply the driveline gets loaded in the opposite direction (drive). On manual transmission vehicles depressing the clutch while in a deceleration immediately releases load on the driveline and may produce a clunk noise as the driveline unloads.
Note:
Compare this complaint vehicle to a like vehicle. If the results are the same, this is a normal condition. For additional diagnostic information, refer to the appropriate Service information.
Whenever there are two or more gears interacting with one another, there must be a certain amount of clearance between those gears in order for the gears to operate properly. This clearance or freeplay (also known as lash) can translate into a clunk noise whenever the gear is loaded and unloaded quickly, or whenever the direction of rotation is reversed. The more gears you have in a system, the more freeplay the total system will have.
The clunk noise that owners sometimes hear may be the result of a buildup of freeplay (lash) between the components in the driveline.
For example, the potential for a driveline clunk would be greater in a 4-wheel drive or all-wheel drive vehicle than a 2-wheel drive vehicle.
This is because in addition to the freeplay from the rear axle gears, the universal joints, and the transmission (common to both vehicles), the 4-wheel drive transfer case gears (and their associated clearances) add additional freeplay to the driveline.
