I also feel this slight rumble/vibration at 30-37 mph, usually when I first start out with the car. It definitely feels like its coming from the rear and first I attributed it to rough asphalt, which I was on at the time. Then it happened when I was on a smooth road. I figured it was a tire slightly unbalanced since it was so specifically happening at only that speed range. Now after reading your posts, I'm not sure. I'll definitely check it out--it doesn't seem any worse now than when I first noticed it and it's not there every time I'm travelling at 30-37 mph, but often enough. Any other suggestions?