Can you reproduce this at home by yarding on the bars, or is this an only while in motion thing? Creaks can sound like they're coming from a lot of places they aren't, remember you're sitting on a giant set of wind chimes somebody welded together, so there's lots of opportunity for resonance.
How much do your bars flex when you really have at it? You could be getting a little creak from the bars not being clamped by the entire width of the stem if your bars are flexing a little, but that's unlikely. Did you lube the contact surface between the stem and its expander wedge? Also, try raising the bars like 3 mm and seeing if that helps, as it might be the stem being jammed against the top nut.
OK, that's all the BS theories I've got, somebody else want a shot at this?