2011 double cab 4 cyl vibration problems

I bought my new truck in may of 2011 and the only issue I'm having is sometimes when I'm at low speeds and braking or not braking my front end vibrates pretty bad almost with the same sound when rotors are warped. Except the truck is brand new and the dealership checked my brake pads and rotors and found no problems...I want to take it back but it doesn't always do it so they can't "duplicate" it any ideas of what it could be so I'll be better prepared when I go back??? thanks in adnavce