Mine is older then yours and mine does what your talking about. I hear many other complaining of the same thing too. I hate to say it.... It may be the nature of the beast.
^My 08' does the same thing. Have yet to do the fix.Your issue is a fairly common one.
Not sure if there is a more current TSB but here's one covering your exact problem on the 2005 and 2006 model years..
Find out where your seatbelt clip is. If it's not in the right spot, it will bump between the seat's movements and the plastic panels in the cab, making noise.I have a new 2010 Frontier bought a month ago. The empty front passenger seat rattles when going over bumps. I brought it to the dealer who balanced the tires. This seemed to fix the problem, however the rattle is back. Could it be something else causing the problem, or would the tires go out of balance after only a week? Something is just not right and it's getting frustrating.