2 reasons for a creak.
I had an overlong rear dropout nut that wouldn't tighten down, this caused a real bad creak. Check that the bolts are tight and when done up they should not spin in the frame. Had to file down the nut to cure.
The pivot bearings can come loose, the one at the top of the shock is a good place to start looking. Simply tighten to remove the creaking. There is a guide on the Marin Web site.