The front output is always engaged, so if the front driveshaft is turning then the front output (and the chain) is turning too. The chain is attached to the sprocket on the main shaft, so that is also turning.
The differential planetary is only engaged in Full-Time mode. The differential is locked in the Part-Time modes, and completely disengaged in 2WD and Neutral (free spin on the needle bearings).
Mine makes a kind of chirpling sound from the gear noise in 2WD, goes away in Part-Time and Full-Time modes.
Popping would be something else, maybe a shift pad is worn out, or the differential teeth are lightly engaged, or something else.