aluminum is definitly a noisey metal. and if it's creaking and the components have been tightened/greased properly (all threads, even small bolts) i wouldn't doubt it's the frame it's self. as PPC said, check the welds, or any stress point near or around the bottom bracket/rear stays area. if that isn't the culprit though, i'd say give the components a one or even two over to make sure they are structurally sound. alu. cranks ,pedals ,disc calipers could cause these things too. good luck