Doesn't make a sound in 2wd but when you lock it in 4x4 it sounds like something is knocking on the bottom of my jeep but still pulls fine, my jeep is lifted 4inches but idk if that has anything to do with it. Anyone have any ideas?

 

U joint in the front shaft. Or pinion bearing on the front diff.

 

sounds like a loose chain in the transfer case.

 

Engine & Transmission mounts.

 

X2
Mine did the same thing. I thought a shock was loose, turned out to be a MM.