The same experience here - my Al framed bike makes more noise than the steel framed one (both are road machines). I believe that one reason for this is that aluminium (aluminum)* is softer than steel, and also has the tendancy to stick where there is friction or a fretting motion between two parts. This is known as "graunching" in elevated technical circles... The creaking is most likely the making and breaking of these small adhesions in the places where two parts are supposedly fixed together e.g. seatpost and seat tube. Of course, no two parts are ever really fixed together on a bicycle!
With regard to carbon fiber, they make loudspeaker cones from the stuff, no wonder its noisy
Hope that is of interest
Ed
* Element number 13, atomic weight 26.98. Discovered by English scientist Humphrey Davy in 1808 and originally named (by him) aluminum. Big slice of humble pie to us Brits!