What is this buzz you speak of? I rode a hilly century last weekend on a new-to-me aluminum frame/carbon fork rig. It's got 'comfort compact' geometry, so slightly longer chainstays and wheelbase than a comparable 'road race' geometry. I swapped the wheels/tires from a Reynolds 531 framed steel bike which is a good 7-10lbs heavier. It's also far more flexy, and feels more sluggish up hills. However, over rough pavement and gravel roads, it's a much cushier ride. Despite this, I couldn't tell much difference in the overall ride. There really wasn't much more buzz in the aluminum frame, but neither did the carbon fork seem to dampen buzz vs. steel. In fact, I think carbon is probably stiffer than my steel forks. I did hit about a mile stretch of milled pavement awaiting resurfacing, and I could detect the increased stiffness of the frame vs. my 531 steel bike. It threw the bike around a bit more. The aluminum/carbon bike isdefinitely nicer up hills with the reduced weight and added stiffness.

I seem to remember your Marinoni has narrower tires than the Roubaix. That makes a huge difference.