Yes thats normal, its a bit more pronounced in winter as the oil viscosity is more and therefor it takes the pump a few seconds more to circulate oil through the rockers and kill the noise. In summer the oil is thinner and shoots around the engine quicker. Its best not to rev it from start up, let it idle for 30 secs to make sure the engine is fully lubricated before driving off. Prolonged rattling may indicate a fault and wants checking out - it is not neccesary to use a chrysler dealer - take to a Merc dealership, they can diagnose it and do any warranty work.