My 93 d21 transfer case makes a weird popping/thud noise when engaged. Any ideas?

stretched chain, Is it making the sounds going into 4x4, or is it only while under way?

Thanks for the help. It only seems to do it while under a load both in 4low and high range. And the strange thing is is sporadic. Speed doesn't seem to affect it.

are you sure it's the tcase?

Not a 100% sure what else could it be? Bad cv? U joint?