Have any of you done an oil change yet? The rattle may be due to the oil filter. Most filters have a drain-back valve to prevent oil from leaking out so when the car is started, the oil pressure comes up quickly. If the filter either does not have the valving (some cheap ones don't, some are smaller with less oil capacity and who knows what they use at the factory) or the valve is stuck open, it can cause rattle on startup. Be sure to insist on a top quality filter when the oil change is done and see if the rattle goes away.