It must be crack somewhere! Have you inspected the usual crack spots?
Otherwise list every pivot, Junction, contact point...bolt, nuts, pin, clamp...and work through it. Clean it then grease/carbon paste/loctite where appropriate until you've nailed it.
Yeah mate, I always head straight for the crack spots.
TBH, I think it might have been a sticky bearing or something.
It began all of a sudden after I hit an average sized washout coming down a hill. CREAK CREAK CREAK. By the time I got home it had stopped. Anyway, I checked things over and couldn't find anything obvious.
Next ride out it started again.
Took it to the LBS who explained (as I already know all too well...) how hard creaks can be to track down. So it was given the once over and came home creak free. It started it again a little bit the other day - cracking the shits, I bunnyhopped the thing hard a few times and it stopped!
So this morning I pulled the back end off and cleaned everything, popped the seals and packed the bearings. Some had more grease than others. Interestingly, the derailleur hanger-frame junction was still as dry as a chip, I assumed the shop would have lubed this being one of those creaky spots. So I lubed that too.
No creaks at this stage though I only took it for a 12km test ride.