It could be the hydraulic lifters. They drain out pretty fast over night and can cause that noise until you have good oil pressure. If I remember right Nissan is well known for that. Like mayer suggested a good oil may take care of this, as well as regular oil and filter changes. Also make sure that you use a good filter. You may also have less of the cold starting noise with the use of a synthetic oil. Do not forget to check that it is not one of the heat shields that may vibrate against the exhaust. Good Luck to you.