1993 Jaguar XJ6 Noisy vibrations.

My 1993 xj6 makes a rumbling noise between the speeds of 35mph and 55mph. It also seems that something is loose near the passenger side rear wheel. It goes away after it reaches approximately 60 mph. What could this possible be? The tires are balanced and there is nothing that I can find. My concern is that this could be a costly situation. I need some advice before I take it to a dealer to repair.