Yea it happens to G37 guys too.
Look around in this
thread a bunch of guys say they have it.
Some claim its from the diff, others say its from engaging to quickly and 'shocking the drivetrain'
Either way the general consensus is that low RPMs + quick clutch from 1-2 makes the clunk.
I don't know if there is remedy or if people are just accepting that it happens. Good luck.