I'd honestly say it's pretty normal...it goes along with all the other NVH that these things have stock. My brand new STI back in 2006 did the same thing. My current STI now also still does the same thing at 118k.
I'd honestly suspect that the stock engine mounts are a bit more firm than an average commuter car (with even firmer upgrades still available), so it will pass along the vibrations of the engine to the rest of the chassis.
Do you notice any fluctuation in RPM when this happens?