My car has a loud roar when it drives at any speed, but not when I rev the engine. At first I thought it was a leak in a hose, which I fixed, and then I thought maybe a hole in the exhaust system, but that wasn't it. Then I wondered if it had to do with a recently changed timing belt. I'm having the tires checked tomorrow to see if there's a belt separation. Can anyone advise as to what the problem might be?