98 Altima Start Up Question

My girl has a '98 Altima, for some reason it doesnt sound right to me. When ever she first starts it up in the morning you can hear the engine knock for like 1 sec, then it seems like the oil gets to circulating because after that the engine runs as smooth as it has since day one. It just doesnt seem right to me because I've never heard this b4 in a car. Does anyone have a clue?

Knocking, well lets see, what kinda maintenance have you given the car.. What kind of oil are you using... Have you had any major engine work done.. well what ill suggest switch up the oil, use syntethic... also check your valves, if you dont know how take them to get check they might need an adjustment.. If you buy a Haynes or Chiltons manual it will tell you how to check the clearance.. hope this helps..