i think i know
I have had a simalar experience with a wide tire car. The front, inner wheel well cover ( that big piece of plastic that protects the engine from tire thrown debris) can get loose, especially if you have run over something big. When you are driving forward, the tire just runs down the edge, and you never hear it. BUT in reverse, the tire tread grabs the leading bottom edge of the cover, and , well, like a a baseball card in your bike tire spokes, it makes a sound. When you turn the wheel in our out, while in reverse, it will get REAL loud. Check it ou. Let me know.