Where? On the road, in a field, in your driveway, where, how fast are you travelling and what tires do you have. Need more info...
As an aside, if you have R1's (like I do) and you drive on hard packed dirt or blacktop, the rear end will 'vibrate' from the tires rotating. I get that all the time. I ignore it. it's the bar tread doing it. If no R1's elaborate a bit more.