On my 2000 Gt 5speed I get a howling noise when I am backing out of my driveway first thing in the morning. Don't know which wheel it is but what could it be if it is even from the wheel? Is it due to the cold or not? Doesn't do it going forward though.
Generally tranny's use straight-cut gears for the reverse. Straight-cut gears are cheaper to make, but are noisy, esp. at higher rpms. This noise is exacerbated when the gears and fluid are cold.
The old-school GM big block musclecars sometimes had the Muncie "rock crusher" 4spd that was famous for making that noise... in all four forward gears! Straight-cut gears are stronger but noisy. Rent, borrow, or steal an old movie called "Two-Lane Blacktop." The 55 chevy has a muncie that whines in all the gears.
I digress. Probably nothing wrong with your machine. Probably your trans instead of the cars rear.