Engine rattles between 1700-2000 RPM

The 3.5 motor has a few hydraulic tensioners that work with timing chains for the variable cam system.
It is a very touchy sytem and any low oil or avoiding the oil change recomendations of the manufacturer will cause them to get dirty and start to let the timing chains get loose and rattle.
I would suggest changing the oil early, 1500 miles with a synthetic or SYN-Blend and maybe even an engine flush, (you can find one by Lucas Brothers at a parts store, and see what happens.
Also, always use an OEM Nissan filter as it has an anti-drainback valve that keeps oil in the top end for start up. This is very important to keep the motor in good shape. Mobil 1 makes a good filter too.
Try getting some fuel system cleaner as it will help the top end as well. Use SEA FOAM or BG44K, as they are the only good fuel system cleaners that really work. Use them just before changing oil as they work so well they will dirty the oil from the deposits they remove.
I hope this helps out.

Clear post

P