My iQ has a rather terrible resonance - makes the car sound growling, and the inside mirror vibrates along - between about 1500 to 2000 rpm. I get it in every gear while accelerating. It only does this under load, not when revving up in neutral. It does it in every gear, at any speed, around this engine speed. It also doesn't seem to do it much when going along at constant speed, but if you then floor the pedal again, it vibrates again. So it's not the tires, because that would be at one speed (