Hey mate, I've had similar noises in the past, there have been two different causes of them.
1) First time when my car was still quite new, the clunk wasn't overly loud but concerning. After checking things I noticed the RH rear tyre was soft (had 10psi) after inflating the clunk disappeared. Thinking had something to do with the rolling diameter difference between LH rear and RH rear.
2) Second time was prior to snapping my bolts on the forward mount for the diff, then the clunk was Huge after the bolts snapped. This occurred around 42,000kms Mazda fitted new bolts and problem is fixed.
3) Third time about a month later, the clunk was soft but same kind of clunk as in instance 2. I checked the bolts when I got home and found they were very loose! Mazda re-torqued them and applied loctite, this occured at 44,000kms and I'm up to 50,000 with no problems.
RE: Transfer case leak I had my seal fixed around 30,000kms and seemed to be ok for a while, but since looking underneath recently I now have another leak
be going in soon for my 50K service so hopefully they fix it (again)
Hope this helps and you work out what causes the "clunk" Kov