it is possible that your tires aren't balanced or they are out of round or flat spotted. If it is the tires or wheels, it would get worse the faster you go. first try the driving back and forth to load and unload the L and R sides. You didn't say what car this was for, if it's an mk3, you have tapered rear wheel bearings that can grind if overtightened or not regreased, if it's an mk4, you have sealed rear wheel bearings.