My '06 C280 4matic (6500 miles) emits a a rumbling or low frequency growling noise between 47 and 53 miles per hour. It seems to come from the rear of the car. Tire rotation did not change the noise. It occurs regardless of the gear I select. The noise is not affected by road conditions or weather. Below and above the speed range the car is smooth and quiet. Any thoughts will be appreciated.