if you notice the vibration is coming from exactly _underneath_ the car, then it might be a heat shield. they put thin pieces of metal over the catalytic converter, because it gets very hot and you don't want to start a fire if you take a turnaround on some dry grass or something.
catalytic converter heat shields are usually tacked on at 4 corners, if one or more of the corners rusts off or otherwise lets go, it's direct metal-on-metal vibration. usually, this is noticed when you're revving the engine, not the stereo, but it might be the problem.
you should be able to turn your engine off, turn the stereo up and locate the vibration. shut the windows, too, so you can concentrate on hearing the vibration (and not the tunes).
typically though, as somebody mentioned, license plates are also notorious for rattles, especially since nowadays they're only held on with 2 bolts instead of 4.
good luck.