2004 Frontier Noise

Hi guys, I have a brand new '04 Nissan Frontier XE V6 Crew Cab Long Wheel Base ( 330 miles) and when I go over bumps or on gravel I hear a slight deep vibration that sounds like it is coming from the back seat or rear of the truck.....it is a vibration similar to bass speakers vibrating when to loud. Any ideas?

I have a 2003 Frontier Crew Cab with the same type of noise. Put you hand on the headliner and apply pressure and the noise will go away....

I believe it has something to do with the roof rack and/or the headliner. I thought it originally was the wind deflector mounted on the roof rack but the noises persist even after removing the wind deflector.