Problem started early on, certainly before 15K miles. Have been to the dealer three times. They don't know what the cause of the problem is, but each time they try something and it ends up costing me money, but the problem doesn't go away. The metallic thump-thump noise is most prominent in the 10-25 mph range. It's still there at highway speeds, but it is partly overcome by the tire/road noise. I've had tires, wheels and brakes checked several times - everything is OK, but the noises persist.