2012 Nissan Altima Just Purchased Altima Start Morning Let Warm Couple Seconds

I just purchased a 2012 Nissan Altima. when I start it up in the morning to let it warm up for a couple of seconds, it makes a knocking sound. Once it warms up and I drive it later on in the day the knocking seems to fade away, but eventually will here the knocking again. Is it normal for a 2012 to make a noise when you start it up? I talked to the Nissan service people and they said, that it's normal for 2012 Nissan Altima and they spoken to the mana factory and the technician. There is nothing to worry about.