sympathetic vibrational rattle coming from passenger side at 60mph, increasing with speed.

The car drives absolutely fine, and there are no noises at all, until you reach 60mph, then a rattle from somewhere around the front passenger side appears, and increases with speed. It stops once speed dips below 60mph. It sounds like something is loose, and is not a metal upon metal sound. more like plastic against plastic. there is a loose fitting under the wheel arch which is attached to the bumper, and is vented, could it be this, or could it be coming from behind the wheel itself. It does not affect the drive of the car, and there are no other symptoms.