I searched around the forums for this and no one seems to have the same problem as me. Sometimes, when I start my car (2008 2.5 S) up (any time of the day) I get this rattle. When I start driving the rattle continues, I can control the loudness of the sound with the pedal, it seems to be loudest around 2000ish RPM, give or take a few hundred. Once I drive for a few minutes, it goes away. Seems to be coming from the engine, but it could be the exhaust. My dad's 11' Rogue has the same problem, his is just louder than mine. Weird thing is, this doesn't happen everytime I start up the car so I'm afraid if I take it to Nissan they won't be able to replicate the problem.