when engine is cold and start driving a vibration/buzzing sound starts at 1500-1900 rpm when driving slow. it sounds like its coming through thedash behind the radio. if you go over 1900rpm or are under 1500 you dont hear it. but as you are driving and the engine is warming up it starts to dissappear. it is only when engine is not warm it is around. any thoughts or does anyone have this problem. truck is a 2011 f150 eco.