Keep being a pain in the a%% with them. My Altima got 23.5mpg today driving around town with a/c on full blast, about 23000 miles. Obviously there is something wrong with yours. Make a case with Nissan Consumer Affairs. Could the rattle have something to do with the timing chain? Mine actually did have a recall for an issue with the heat shield and the dealer's work fixed it.
http://www.nissanusa.com/apps/contactus
Nissan Consumer Affairs
P.O. Box 685003
Franklin TN 37068-5003
(800) NISSAN-1 (or 800-647-7261)
8:00 a.m. - 5:00 p.m.,
Eastern / Central Time / Pacific Time
Monday through Friday