Goes away once the engine is warmed up... You gave a key piece of info there. Here is what I think is going on: when you fire the engine first thing in the morning, the car goes into "open loop" running, where the fuel injection system does not use the primary O2 sensors to determine the fuel air mixture. The engine uses some predetermined tables stored in the ECU to inject a predetermined amount of fuel based on outside air temp, but much of this atomized fuel since the engine is cold, condenses on the cylinder walls before actual ignition/combustion so warm up requires the injection system to provide more fuel than usual at idle. The colder the engine, the more fuel is injected into the cylinder to compensate for the atomized fuel condensing on the cylinder walls, also known as washing the bores (of oil). The colder it is, the more fuel is injected to maintain an idle, and more engine noise from combustion results, plus more vacuum, more air rushing through the ISV, etc, etc etc, during warm up.
After the O2 sensors come up to temperature and the engine temperature increases, the engine switches from "open loop" to "closed loop" where now the electronics are relying on the primary O2 sensors to determine the fuel air ratio. Once the engine is warmed up, the fuel no longer condenses on a cold cylinder wall, less fuel is used and the engine at idle is quieter in operation.
So there you have it.