Yeah your oil is either worn/dirty or the wrong viscosity. Quite common for Nissan petrol's to have a little rattle on startup if the oil is not right. My bluebird and maxima did it and the VL commos were renowned for it. My 4.8 has done it once or twice on a cold start when it was due for an oil change, change the oil and all good again.
As far as the high idle goes, on a cold start it should jump up to about 1500-1700rpms straight after start and then quickly drop down to about 1100rpms and then slowly slowly drop down to about 750rpms. You can also re-tune the idle without plugging it in to a computer if you follow the service manual instructions, but I've never had to do that to mine.