A lot of the noise is from the tires... esp. if you have the stock Continental 4x4 Contact's. It's especially pronounced on wet roads. Replacing the stock tires can make a huge difference. Check out our sponsor
http://www.tirerack.com for tire reviews. Some of members have tried the Yokohama Avid TRZ with good results. My wife's car (Mazda Protege) has the Kumho Solus KH16. This is a nice 3 season tire (The Solus is not a particularly good winter tire from the reviews I've read) that is very quiet and smooth riding. It's available in sizes that fit the Rogue SL models: 225/60R17. I am seriously considering them for my Rogue as I do lots of highway driving. They are very competitive price wise and I have dedicated winter tires.
As for the engine, the Rogue has a large displacement 4 cyl motor. It's never going to be as smooth or as quiet as a V-6. Generally you don't hear the engine much when cruising, just during acceleration. I would say the Rogue's engine noise is about average for it's class. There are smoother, quieter motors out there... and there are others that are far worse.