Unless your seat post was a .1mm too small, you shouldn't have any rattle there. Maybe a creak in some odd situations, but certainly not a rattle.

My feeling is chain-slap against your driveside chainstay. Try putting it in the Big-Big combination for a minute and see if the noise persists. This will both tension up the cables and the chain, isolating them as much as possible from being the culprit.

Also, check that you don't have cable housing rattling somewhere near the head tube. Odd bike noises have an weird way of sounding like they are in a totally different area than where the problem actually is. Lastly, check that your stem is properly tightened. (Loosen top cap bolt then pinch bolts, tighten top cap bolt then pinch bolts).